Tools

Cost of Floating a Refundable Sum to a Deadline

You pay a sum now and get it back later, unless a deadline passes first. What does the wait cost, and what does missing it cost?

The decision this page was built for, in the words it was recorded in: to get the house I want I have to complete on it before mine sells, which means paying the additional-property surcharge and reclaiming it later. What does floating that money cost me, and what happens if my sale slips past the deadline? The calculation below is that question with the property, the country and the surcharge taken out of it, because none of the three changes the arithmetic and all three would have limited it to one place.

Two things are true of a sum you pay now and get back later, and they point in opposite directions. Funding it costs a little every month, which is the figure a lender will quote you. And the whole of it disappears if a window closes before the money comes back, which is a figure nobody quotes at all. The second is not a larger version of the first: it does not grow while you wait, it does not grow with your rate, and it is not reduced by having nearly made it. On an ordinary arrangement it can be worth eight years of the monthly figure, and one month of slippage can cost a hundred times what that month of funding cost.

What this tool does not decide

Any of the six numbers. The sum, the rate, the arranging cost and the length of the window are all yours, and no control on this page carries a default. That is not caution: it is the whole reason one page can be correct wherever this arrangement exists rather than correct in one country.
How likely you are to make the deadline. There is no field for it and no blended figure built from one. You do not have that probability, nobody publishes it, and a cost that averaged the two outcomes would be true of neither. Move the month instead and look at both sides of the boundary yourself.
What the money would have earned elsewhere. The rate you enter is what funding the sum costs you. An excess return over that is a forecast, and this page makes none.
Whether to enter the arrangement at all. A cost is not a recommendation. What a float buys you depends on what happens if you do not have it, and that is not in this arithmetic.
Anything about tax, in any jurisdiction.

Your figures

The interactive calculation runs in your browser and needs local JavaScript, which is currently switched off. Activating the Calculate button does nothing at all: no request is sent and nothing is submitted.

Everything else on this page works without it. The model, the formulas, the timing convention, the worked example, the assumptions and the limitations are all in the methodology below, and the companion explainer covers the mechanism in full.

Your arrangement, in six numbers

All six figures are yours and this page publishes none of them. It names no country, no tax, no scheme and no window that anybody legislates, and there is no field that could carry one: what the sum is, why it is refundable and how long the window runs belong to whatever arrangement you are actually in. Take the deadline from the authority you are dealing with and the rate from whoever is funding you. Nothing you type is sent anywhere.

The sum, and what funding it costs

The sum you pay now and expect back later

The whole of the money that goes out now and is supposed to come back. It is also, exactly, what you lose if the window closes first, which is why it appears twice in the result. There is no currency here and no field for one: every amount on this page comes back in the unit you enter this in.

What funding that sum costs you for a year

What it costs to have that money out, as an annual rate compounding monthly. Enter 0 if you are funding it from cash at no stated cost. That is a real arrangement rather than a trick answer, and it is the one that shows the cliff is a property of the refund and not of the borrowing. This is not what the money might have earned elsewhere: an excess return over a funding cost is a forecast, and there is no field here for one.

The one-off cost of arranging the funding

Fees, valuations, legal work — whatever it cost to put the funding in place, once. Enter 0 if there was none. It is not refundable, so it lands on both outcomes equally and changes neither the difference between them nor either of the two comparisons beneath the result.

The two months, which are the whole of the risk

Whole months until you expect the sum back

Counted from the month you pay it, so 0 means it comes back the same month. Enter the month the money is in your hands rather than the month you become entitled to it: whatever processing time stands between the two is part of the wait. A figure past the deadline below is not an error here — it is the case this page exists to price, and it is where your own view of the risk belongs.

The last whole month in which it can still come back

The end of the window, counted the same way. This page publishes no window and has no field that could carry one: how long yours runs belongs to whatever arrangement you are actually in, and it is the figure to take from the authority you are dealing with. Zero is refused, because a sum that can never come back is not a float — enter it as the arranging cost above and float nothing.

How much of it to draw

How many months to draw on the chart

This is the only control that changes no figure. Every number in the result is computed at your own month whatever you draw here, and the chart is free to stop short of the deadline or run past it. A month outside the range you ask for is simply not marked, rather than being pinned to the edge of the picture and made to look like it falls there.

Result

Missing the deadline costs exactly the sum, at every month and at every rate. That is not a rule of thumb. The two outcomes differ in one respect only, which is whether the sum comes back, so the funding cost you have already paid appears in both of them and cancels out. The cliff therefore does not grow the longer you wait, does not grow with the rate you are paying, and is not made smaller by having nearly made it. A month late and three years late cost the same cliff. That is the number a monthly carrying cost hides, and it is why both outcomes are published side by side here rather than one of them.

Enter your six figures above and select Calculate. Nothing is sent anywhere: the calculation runs in this browser, and no value is stored, shared or placed in the address bar.

Both outcomes, at your own month

Three rows and never one. The cost of the outcome you expect is published beside the cost of the one you do not, because the finding is the distance between them rather than either figure on its own.
Which outcome	What it costs
What it costs if the sum comes back	Not yet calculated
What it costs if it does not	Not yet calculated
The difference between them	Not yet calculated

The two outcomes, month by month

Two lines, one for each outcome, drawn over the months you asked for. The lower line is what the float costs if the sum comes back. The upper one is what it costs if it does not. Both rise, because funding the sum costs more the longer it is funded. The distance between them never changes: it is the sum, at every month on the chart. The deadline is marked, and so is the month you entered.

The chart appears here once you calculate. Every figure it draws is also published above it as a number, so nothing on this page depends on being able to see it.

What the float costs

What funding the sum has cost by then: Not yet calculated
The one-off cost of arranging it: Not yet calculated
What the float has cost you in total: Not yet calculated
What missing the window would cost, on top of all of that: Not yet calculated

The same cliff, in other units

How many months of carry the cliff is worth: Not yet calculated
How many times the carry to the deadline the cliff is: Not yet calculated
The monthly rate your annual one comes to: Not yet calculated
What that compounds to over a year: Not yet calculated

Where you stand

Months of room between your month and the deadline: Not yet calculated
Where that leaves you: Not yet calculated
What is happening to the funding cost: Not yet calculated

There is no field here for the chance that your sale completes in time, and no blended figure that weighs the two outcomes by it. A blended cost would be larger than one outcome and smaller than the other, which makes it true of neither world you can actually end up in. It would also hide the finding: a weighted average moves smoothly as the chance moves, and the fact worth knowing is that your own outcome does not move smoothly at all. It steps, by the whole sum, in one month.

Every figure here is a cost of carrying the sum: what funding it accrues, plus what arranging that funding cost, plus the sum itself in the outcome where it does not come back. It is not a return, it is not a profit, and it is not the price of anything you are buying. The rate you enter is what the money costs you, so what it might have earned somewhere else is not in this arithmetic and cannot be added to it.

This is arithmetic on six numbers you supplied. It assumes the sum comes back whole or not at all, that the funding cost rolls up rather than being paid monthly, and that the rate stays where you put it for the whole wait. An arrangement that returns part of the sum, pays interest on the refund, or repays it in instalments is a different calculation and this one would be wrong about it. Whatever processing time stands between the event that entitles you to the refund and the money arriving is part of the wait, so enter the month the money is in your hands rather than the month you become entitled to it.

Neither figure is advice about whether to enter the arrangement. What a float is worth to you depends on what you are buying with it, on what happens if you do not, and on how confident you are about a date this arithmetic has no opinion about. This page prices both outcomes and weights neither, because a reader in this position does not have the probability that would do the weighting and nobody publishes one. Your own view of the risk belongs in the month you enter, and nowhere else.

How this is worked out

Why the deadline matters more than the months

Funding a sum for longer costs more. That alone is compound interest and needs no page. What makes this different is that the sum is refundable up to a point and then not, so there are two outcomes rather than one, and they are not near neighbours.

The two outcomes differ in exactly one respect, which is whether the sum comes back. Everything else is identical: the same funding cost has accrued in both, and the same arranging cost was paid in both. So when you take one away from the other, all of that cancels, and what is left is the sum itself.

Three things follow, and each of them is the opposite of what a monthly carrying cost suggests. The gap does not grow the longer you wait. It does not grow with the rate you are paying. And it is not made smaller by having nearly made it: a month late and three years late cost the same gap.

One arrangement, one month apart

These are the calculation model's own canonical figures, at its own canonical arrangement: a sum of 20,000 floated at 9% a year with 1,500 of arranging cost, against a window closing at month 36. They are a worked example of the mechanism and not a suggestion about anybody's numbers.

The whole finding in two rows. The extra month of funding cost 196.30. The slippage cost 20,196.30, of which 20,000 is the sum. The funding cost is under one per cent of the damage.
When the money arrives	What the float cost	Why
Month 36, the deadline month	7,672.91	The sum comes back. What the float cost is the funding it accrued plus what arranging it cost.
Month 37, one month later	27,869.20	The sum does not come back. One further month of funding cost 196.30; the slippage cost 20,196.30.

The arithmetic

Six numbers you supply, and nothing derived from anywhere else. The rate is divided by twelve to get a monthly one and compounded month by month; the two outcomes are the same line, one of them with the sum added back.

i                    = annual rate / 12

carry(t)             = sum x ((1 + i)^t - 1)

if it comes back     = arranging cost + carry(t)
if it does not       = arranging cost + carry(t) + sum

the difference       = sum                          at every t, at every i

months of carry      = ln 2 / ln(1 + i)             how long carry takes to reach the sum
multiple of carry    = sum / carry(deadline)        the sum against the carry to the window

The last two lines are the same gap said in units you have been thinking in. At 9% a year the funding cost takes about 93 months to reach the size of the sum, which is to say that missing the window costs what nearly eight years of funding it costs. A reader looking at a monthly figure of under two hundred is looking at the smaller of the two numbers on this page.

The three positions, and why the middle one is separate

Arriving in the deadline month costs exactly what arriving early costs. It is listed separately because the figures are the same and the exposure is not.
Where you are	What the float costs	What that leaves you exposed to
Inside the window	Funding cost plus arranging cost	The sum comes back. The cliff is what you avoided, and it is the same size however early you are
Exactly on the deadline	Identical to the row above, at the same month	No room left. One month of slippage moves you to the row below, and nothing about the arithmetic warns you first
Past the deadline	Funding cost, plus arranging cost, plus the sum	The sum does not come back. How late you are changes the funding cost and not the sum

Why there is no chance of making it, as a number

The field this page most obviously lacks is the probability that your sale completes in time, and a blended cost that weighs the two outcomes by it. It is refused, and the reason is not caution. A blended figure is larger than one outcome and smaller than the other, which makes it true of neither world you can actually end up in. It would also hide the thing worth knowing: an average moves smoothly as the chance moves, and your own outcome does not move smoothly at all. It steps, by the whole sum, in one month.

Your view of the risk belongs in the month you enter. Move it, recalculate, and you can see both sides of the boundary for yourself, which is a more honest way of exploring a probability than being handed the arithmetic of one you did not supply.

What this does not model

any probability that the deadline is met, and any cost that weighs the two outcomes by one
what the money would have earned had it not been tied up
a refund that comes back in part, in instalments, or with interest paid on it
a rate that moves, a facility redrawn, or funding costs settled monthly rather than rolled up
the cost of failing to complete the purchase the float was arranged for
a price reduction taken to sell inside the window, and what that trade costs
any country, tax, surcharge, scheme, statutory window or reclaim procedure
any lender, product, currency or calendar date
whether to enter the arrangement in the first place

The limits of the calculation

Every figure is yours. This page publishes no sum, no rate, no arranging cost and no window, and there is no field that could carry one. That is not caution: it is what makes one surface correct wherever this arrangement exists rather than correct in one place.
The sum comes back whole or not at all. An arrangement that returns part of it, returns it in instalments, or pays interest on the refund is a different calculation, and this one would be confidently wrong about it.
The funding cost rolls up rather than being paid as you go, and the rate stays where you put it for the whole wait. A facility you service monthly, one that is redrawn, or a rate that moves is a different arrangement.
There is no probability anywhere in this arithmetic. Both outcomes are priced and neither is weighted, because you do not have that probability and nobody publishes one. Your own view of the risk belongs in the month you enter, and moving that month is how you explore it.
The rate is what the money costs you, not what it might have earned. An excess return over a funding cost is a forecast, and this page has no field for one and no view about one.
Nothing here says whether the arrangement is worth entering. What a float buys you depends on what happens if you do not have it, and none of that is in six numbers.

A sum you own and cannot reach is the subject ofa separate explainer on why net worth is not spendable money. What this page adds to it is a date after which the money stops being yours at all.

Go deeper

The distinction this page prices
Why Liquidity Is Not the Same as Net Worth
Why an asset can add to your net worth and still be no help in an emergency, what access time and sale friction actually cost, why a usable share is your assumption rather than a valuation, and why holding more cash has a price of its own.
A refundable sum is yours for the whole of its life and available to you for none of it. That explainer is about an emergency and this page is about a deadline; what they share is the asset you cannot spend while you own it, and the funding cost here is exactly the price of that gap.
Read the explainer: Why Liquidity Is Not the Same as Net Worth

Calculation model and corrections

Calculation model: Refundable Float v1.0
Last reviewed: 31 August 2026
Why the deadline matters more than the months: Because the two outcomes differ in one respect only, which is whether the sum comes back. The funding cost you have already paid appears in both of them and cancels, so what missing the window costs is the sum itself — at every month, at every rate, and whether you are one month late or three years late
Whose figures these are: Yours, all six of them. This page publishes no sum, no rate, no arranging cost and no window, and there is no field that could carry one. That is what makes the tool universal: the arrangement is the same wherever it exists and the numbers are the part that is not
What the rate is: What funding the sum costs you for a year, nominal and compounding monthly. It is not what the money might have earned elsewhere. An excess return over a funding cost is an expected return, no authority publishes one, and there is deliberately no field for it
Why there is no probability: Because you do not have it and nobody publishes it. Both outcomes are priced and neither is discounted by a likelihood: a blended cost would be larger than one and smaller than the other, which makes it true of neither world you can end up in. Your own view of the risk belongs in the month you enter
What a month is: A whole month, counted from whenever the sum is paid. There is no calendar in this arithmetic, no date and no currency. Month zero is the month you pay, and a refund in month zero costs nothing but the arranging cost
How the funding cost accrues: On the sum, compounding monthly, and settled at the end rather than serviced as you go. A facility you pay down monthly, one that is redrawn, or a rate that moves is a different arrangement and this page would be wrong about it
What the deadline is compared against: The month the money is in your hands. Any processing time between the event that entitles you to the refund and the money arriving is part of the wait, and a reader who enters the earlier month gets a correct answer to a question they did not ask
Why a deadline of zero is refused: Because a sum that is never reclaimable is not a float. It is a cost, and the honest place for it on this page is the arranging cost, with nothing floated at all
How far the chart is drawn: As far as you ask, and it changes no figure. The chart may stop short of the deadline and may run past it; every number above it is computed at your own month either way. A month outside the drawn range is not marked, rather than being pinned to the edge
The unit: Whatever unit you entered the sum in, throughout. This page names no currency and there is no field for one
Excluded: Every country, tax, surcharge, scheme, statutory window and reclaim procedure; every lender, product and calendar date; any probability that the deadline is met; what the money would otherwise have earned; a refund that comes back in part or with interest; and any view on whether to enter the arrangement
Rounding: Display only; intermediate values remain unrounded

Correction history

The figure this page calls the cliff is published as the sum itself and is never worked out by subtracting one outcome from the other. Generating the canonical vectors found five months, at a rate of twenty-four per cent, where subtracting the two branch costs did not return the sum but the sum plus or minus one unit in the last place a computer can hold. Nothing about the model is wrong: adding the sum to a large accrued cost and taking it away again does not land back on the same number, and the rounding belongs to the subtraction rather than to the arithmetic. So the figure a reader acts on is the sum, and it carries no rounding at any rate or over any horizon.
The independent verification asserts that difference to a relative tolerance rather than to exact equality in ordinary arithmetic, and asserts it exactly only in sixty-digit fixed point. That is a deliberate narrowing rather than a loosened check: a verifier that demanded two subtractions agree bit for bit would have been asserting a property of how computers store numbers and calling it a property of this model.