Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Result
Extra interest cost of saving alongside
$720
Cash saved by payoff date
$5,425
Months of expenses covered
1.6
Payoff with the split
26
Payoff if all surplus goes to debt
17
Total interest with the split
$2,420
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How to use this
- 1Enter high-rate debt balance ($).
- 2Enter debt apr (%).
- 3Enter monthly surplus available ($).
- 4Enter share sent to savings (%).
- 5Enter essential monthly expenses ($).
- 6Enter savings apy (%).
- 7Enter cash already saved ($).
- 8Read your extra interest cost of saving alongside on the right — it updates as you type.
- 9Hit Share to keep the scenario or send it to someone.
About this calculator
Paying high-rate debt beats almost any guaranteed return, which argues for throwing everything at the balance — until an unexpected car repair puts the whole amount straight back on the card at the same rate. The practical answer is a sequence: a small starter cushion, then aggressive payoff, then a full emergency fund. This calculator models a split of your monthly surplus, showing payoff date and interest cost at each allocation, how many months of expenses you would have banked, and the interest cost of holding that cushion. Use it to find the smallest cushion you can live with rather than the theoretically optimal one.
Worked example
Using the values the calculator loads with:
Inputs
- High-rate debt balance: 8500 $
- Debt APR: 22.9 %
- Monthly surplus available: 600 $
- Share sent to savings: 30 %
- Essential monthly expenses: 3400 $
- Savings APY: 4 %
- Cash already saved: 500 $
Results
- Extra interest cost of saving alongside: $720
- Cash saved by payoff date: $5,425
- Months of expenses covered: 1.6
- Payoff with the split: 26
- Payoff if all surplus goes to debt: 17
- Total interest with the split: $2,420
What each field means
Inputs
- High-rate debt balance ($)
- The high-rate debt balance used in the calculation, measured in $. Starts at 8500 $ so you have a working example on load. Accepted range: 0–500000 $.
- Debt APR (%)
- The debt apr used in the calculation, measured in %. Starts at 22.9 % so you have a working example on load. Accepted range: 0–40 %.
- Monthly surplus available ($)
- The monthly surplus available used in the calculation, measured in $. Starts at 600 $ so you have a working example on load. Accepted range: 0–50000 $.
- Share sent to savings (%)
- The share sent to savings used in the calculation, measured in %. Starts at 30 % so you have a working example on load. Accepted range: 0–100 %.
- Essential monthly expenses ($)
- The essential monthly expenses used in the calculation, measured in $. Starts at 3400 $ so you have a working example on load. Accepted range: 1–100000 $.
- Savings APY (%)
- The savings apy used in the calculation, measured in %. Starts at 4 % so you have a working example on load. Accepted range: 0–10 %.
- Cash already saved ($)
- The cash already saved used in the calculation, measured in $. Starts at 500 $ so you have a working example on load. Accepted range: 0–500000 $.
Results
- Extra interest cost of saving alongside
- Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Cash saved by payoff date
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Months of expenses covered
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Payoff with the split
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Payoff if all surplus goes to debt
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Total interest with the split
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How big should the starter cushion be?
A common rule is one month of essentials or around $1,000–$2,000 before attacking high-rate debt, then rebuilding to three to six months afterwards.
Why not save everything first?
Because a 23% APR balance grows faster than any savings account pays. The cushion buys insurance against re-borrowing, not returns.
Does a 0% card change the answer?
Yes. If the debt costs nothing during a promo window, saving is cheaper — but plan for the balance that remains when the promo ends.
Should retirement matching come first?
Usually yes. An employer match is an immediate return that typically beats even high-rate debt payoff on the matched portion.
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Emergency Fund vs Debt Payoff Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/emergency-fund-vs-debt-payoff-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/emergency-fund-vs-debt-payoff-calculator" target="_blank" rel="noopener">Emergency Fund vs Debt Payoff Calculator — RevenueLab</a> (2026).</p>
Source: [Emergency Fund vs Debt Payoff Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/emergency-fund-vs-debt-payoff-calculator) (2026).