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Lump Sum Needed for a Target Payment

Every other calculator starts with a lump sum and tells you the new payment. This one runs backwards: name the monthly payment you can live with, and it works out how much principal you would have to pay down to get there.

Works offline No sign-up Nothing stored — refresh clears it
Outstanding balance, not the original loan amount.
Exclude taxes and insurance — a recast does not change those.
Lump sum you would need to pay down
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Principal payment that produces your target payment.
Balance after the payment
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Share of the current balance: —
Monthly saving versus today
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Compare this with the servicer’s recast fee.
Estimates only — principal and interest. Taxes, insurance and mortgage insurance are unaffected by a recast and are not modelled. Your servicer’s figures will differ slightly because of the effective date.

Most people arrive here with a number in mind — "I need this payment down to about $1,800" — and no idea whether that requires $20,000 or $200,000. The arithmetic is simpler than it looks, because on a fixed-rate mortgage the payment is exactly proportional to the balance. That single fact turns a vague ambition into a figure you can compare against your savings.

The rule that makes this easy

A fixed-rate mortgage payment is a constant multiplied by the outstanding balance. Change the balance by 15% and the payment changes by 15%. Not approximately — exactly, to the cent, as long as the rate and the remaining term stay the same, which is precisely what a recast does.

So the sum is one line: the lump sum needed equals the balance multiplied by the percentage you want the payment to fall. A payment that needs to fall from $2,098 to $1,784 is a 15% cut, and 15% of a $320,000 balance is $48,000.

Everything else on this page is about why that clean number is not the whole decision.

Why small targets need big money

The proportionality cuts both ways. Trimming a payment by 5% sounds modest and costs 5% of the balance — $16,000 on a $320,000 loan. Trimming it by a third costs a third of the balance. There is no leverage in a recast: you are buying payment relief with equity, at par.

That is worth sitting with before you set a target. If the goal is "a payment I can afford", the target is probably driven by a real budget constraint, and the answer may be uncomfortable. If the goal is "a round number that feels better", the honest answer may be that the cash is worth more in reserve.

The comparison to make: every dollar of lump sum buys the same monthly relief whether you recast or not — but only the recast converts that relief into a permanently lower required payment. Extra principal keeps the payment and shortens the loan instead.

Check the servicer's minimum before anything else

A target payment can demand a lump sum below your servicer's floor, in which case the recast cannot be processed regardless of the arithmetic. Minimums commonly sit between $5,000 and $10,000, with some servicers using a percentage of the balance instead, and one large servicer publishes no minimum at all.

The reverse problem is more common: the lump sum needed is larger than the minimum by a wide margin, and the real question is whether you have that much cash without touching your emergency fund.

Why your target may be the wrong number

The calculator works on principal and interest, because that is the only part a recast changes. Your actual debit also includes escrow for taxes and insurance, plus mortgage insurance if you have it. A target built from your bank statement will therefore overstate the principal-and-interest figure by exactly the escrow and insurance components.

On a $2,578 payment with $480 of escrow, the principal-and-interest portion is $2,098. Aiming for "$2,250 total" really means aiming for $1,770 of principal and interest — a materially different and much larger lump sum. Subtract escrow first, or you will set a target that looks achievable and is not.

Adding the fee to the total

The lump sum is not the full cost of the transaction. Servicers charge a fee, usually $150 to $500 and occasionally nothing, and a few deduct it from the money you send — which means the principal reduction is slightly smaller than the transfer and the resulting payment misses your target by a dollar or two.

Add the fee to the amount you plan to send, and confirm with the servicer whether it is deducted or billed separately. The main recast calculator will show you the exact payment for whatever principal reduction actually lands.

If the number is too big

Three responses are worth considering, in order of how often they are the right one.

Lower the ambition. A payment cut of 10% instead of 20% halves the cash requirement, and if the budget pressure is moderate that may be enough.

Change the rate instead. If your rate is materially above the market, a refinance buys payment relief without spending equity — the closing costs are real, but they are thousands rather than tens of thousands, and you keep the cash.

Do nothing and keep paying. If the payment is affordable and the goal was to feel better about it, the honest answer is that the cash is doing more good in reserve than in the house.

What this cannot tell you

It cannot tell you whether your servicer allows recasts on your loan, whether your lump sum clears their minimum, or what the effective date will be — all three are policy, not arithmetic. It also assumes a fixed rate: on an adjustable-rate loan the payment will be recalculated at the next adjustment regardless of what you do today.

What it can do is replace a guess with a number, which is the part most people are missing when they call a servicer.

Frequently asked questions

Reverse-calculator questions, and what the answer does not cover.

Q
How do I calculate the lump sum needed to hit a target payment?
On a fixed-rate loan the payment is proportional to the balance, so the lump sum equals the balance times the percentage reduction you want. To go from $2,098 to $1,784 is a 15% cut; 15% of a $320,000 balance is $48,000. The calculator does the same sum from the full amortisation formula, which matters only if the rate or term changes.
Q
Does the payment fall in exact proportion to the principal I pay down?
Yes, exactly — provided the rate and remaining term are unchanged, which is what a recast guarantees. A 10% balance reduction produces a 10% lower principal-and-interest payment. That is why recasting offers no leverage: you are buying relief at par, one dollar of cash for one dollar of balance.
Q
Should I use my total mortgage payment as the target?
No. Use principal and interest only. Taxes, insurance and mortgage insurance sit outside the recast and do not change when the balance does, so a target taken from your bank statement will overstate the achievable figure by those components. Subtract escrow first, then enter what remains.
Q
What if the lump sum I need is more than I have?
Three options. Lower the target — halving the payment cut halves the cash needed. Change the rate with a refinance, which buys relief without spending equity. Or accept the payment and keep the cash, especially if the shortfall would come out of an emergency fund you might genuinely need.
Q
Does this account for the servicer's recast fee?
It does not, and neither does the target. The fee is charged on top and is usually $150 to $500; if the servicer deducts it from the money you send, the principal reduction is smaller and the resulting payment is a dollar or two above your target. Add the fee to the transfer, or ask to be billed separately.
Q
Can I use this for an adjustable-rate mortgage?
Only as a snapshot. The calculation is correct for today's rate, but an ARM payment is recalculated at each adjustment, so the target you hit now will be overwritten at the next reset. On an ARM, extra principal is usually the better tool unless the required payment genuinely has to fall immediately.
Q
Does the calculator send my loan details anywhere?
No. Everything runs in your browser in plain JavaScript, there is no server, and nothing is written to your device — refresh and every input resets. That is deliberate: a calculator that collects balance figures and asks for an email has a business model, and it is not advertising.