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
Total interest paid over horizon
$43,697
Break-even month (fix pays for itself)
5
One-time cost to fix now
$7,200
Current monthly interest cost
$1,620
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How to use this
- 1Enter hours to fix properly (principal).
- 2Enter changes touching this code per month.
- 3Enter extra hours per change due to debt.
- 4Enter loaded developer hourly rate ($).
- 5Enter monthly compounding (debt worsening) (%).
- 6Enter time horizon to evaluate (months).
- 7Read your total interest paid over horizon on the right — it updates as you type.
- 8Hit Share to keep the scenario or send it to someone.
About this calculator
Technical debt behaves like financial debt in a specific, useful way: it has a principal (the one-time cost to fix it properly) and an ongoing interest payment (the recurring extra time every future change on that code costs because of the shortcut), and left unpaid, some forms of debt compound as more code gets built on top of the fragile foundation. This calculator estimates monthly interest as the extra hours per change caused by the debt, multiplied by change frequency and developer hourly rate, and compares cumulative interest paid over a time horizon against the one-time principal cost of fixing it now, showing the break-even point where paying down the debt becomes cheaper than continuing to service it. This reframes a common, often-losing argument ('we should refactor this') into a financial comparison engineering leads can bring to a roadmap prioritization conversation with a real payback period attached.
Worked example
Using the values the calculator loads with:
Inputs
- Hours to fix properly (principal): 80
- Changes touching this code per month: 6
- Extra hours per change due to debt: 3
- Loaded developer hourly rate: 90 $
- Monthly compounding (debt worsening): 1 %
- Time horizon to evaluate: 24 months
Results
- Total interest paid over horizon: $43,697
- Break-even month (fix pays for itself): 5
- One-time cost to fix now: $7,200
- Current monthly interest cost: $1,620
What each field means
Inputs
- Hours to fix properly (principal)
- The hours to fix properly (principal) used in the calculation. Starts at 80 so you have a working example on load.
- Changes touching this code per month
- The changes touching this code per month used in the calculation. Starts at 6 so you have a working example on load.
- Extra hours per change due to debt
- The extra hours per change due to debt used in the calculation. Starts at 3 so you have a working example on load.
- Loaded developer hourly rate ($)
- The loaded developer hourly rate used in the calculation, measured in $. Starts at 90 $ so you have a working example on load.
- Monthly compounding (debt worsening) (%)
- The monthly compounding (debt worsening) used in the calculation, measured in %. Starts at 1 % so you have a working example on load. Accepted range: 0–20 %.
- Time horizon to evaluate (months)
- The time horizon to evaluate used in the calculation, measured in months. Starts at 24 months so you have a working example on load.
Results
- Total interest paid over horizon
- 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.
- Break-even month (fix pays for itself)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- One-time cost to fix now
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Current monthly interest cost
- 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 do I estimate 'extra hours per change' honestly?
Look at your last 5-10 pull requests or tickets that touched the debt-laden code and compare actual time spent against a comparable change in clean code elsewhere in the codebase — the delta is your extra-hours estimate. If you don't have a clean comparison, ask the engineers who did the work to estimate the overhead directly; their gut estimate is usually more accurate than an outside guess.
What kinds of technical debt actually compound versus staying flat?
Debt in shared, frequently-touched code (a core data model, an auth system, a widely-imported utility) compounds because every new feature built on top inherits and often worsens the problem. Debt in isolated, rarely-touched code (an old admin tool nobody extends) mostly stays flat — set the compounding input near zero for isolated debt and use a higher value only for debt in actively-growing shared code.
If break-even is past my time horizon, does that mean don't fix it?
Not necessarily — a break-even beyond the horizon means the fix doesn't pay for itself within that window on pure hours-saved math, but it ignores harder-to-quantify factors like bug risk, onboarding friction for new engineers, and morale cost of working around known-bad code. Use the calculator as one input to prioritization, not the sole deciding factor, especially for debt with high failure-risk consequences.
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.
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Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Technical Debt Interest Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/technical-debt-interest
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/technical-debt-interest" target="_blank" rel="noopener">Technical Debt Interest Calculator — RevenueLab</a> (2026).</p>
Source: [Technical Debt Interest Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/technical-debt-interest) (2026).