The formula
How to calculate debt avalanche
The debt avalanche orders debts by interest rate rather than by size. Mathematically it is the optimal method: every spare pound goes where it stops the most interest, so the total cost of clearing the debt is the lowest available.
Rate order and balance order rarely match, which is why the two popular methods differ. A £6,400 card at 27.9% costs far more to carry than a £1,800 card at 22.9%, even though the smaller one would clear sooner.
The inputs, one by one:
- Highest-rate debt balance
- Its interest rate (% APR)
- Its minimum payment
- Extra you can pay each month
Results appear immediately — there is nothing to submit. Changing a field rewrites the link, so you can share the exact scenario you are looking at.
The order the fields are filled in makes no difference to the result — the calculator recomputes the whole formula from whatever is currently in every field, not step by step. That means it is safe to adjust one number, watch the result change, and adjust it back, without worrying about resetting anything first.
Why debt avalanche matters
The formula behind debt avalanche is standard and has not changed in decades; what changes is the situation it gets applied to. Two households can run the identical calculation and land on very different conclusions once their own numbers — income, rate, term, balance — are dropped in, which is why a generic textbook example is less useful than a calculator you can adjust to match your own circumstances.
Beyond a one-off check, the same calculation is worth revisiting whenever the underlying numbers change — a new interest rate, a change in income, a different term. Because the figures live in the page's own web address, coming back to update just one field and compare the new result against the old one takes seconds rather than starting again from a blank page.
The reason a page like this exists at all, rather than leaving the calculation to a spreadsheet or a textbook appendix, is that the formula behind debt avalanche is fiddly enough to get wrong by hand but not complicated enough to need specialist software. That middle ground — real enough maths to matter, simple enough to check instantly — is exactly what a dedicated calculator is for, and it is why the same figure recalculated here should match a careful manual calculation almost exactly.
A calculator like this one is often bookmarked and returned to repeatedly over months rather than used once, particularly for anything tied to an ongoing plan such as a mortgage, a savings goal or an investment being tracked. Because the figures live in the web address rather than only in memory, coming back to the same page with updated numbers is quicker than starting from a blank spreadsheet each time.
Worked example
Work through the defaults on this page:
- Highest-rate debt balance: 6,400
- Its interest rate: 27.9 % APR
- Its minimum payment: 160
- Extra you can pay each month: 250
That gives:
- Months to clear with the extra: 19.6 months
- Months on the minimum alone: 115.7 months
- Interest saved: 10,469.3
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
The comparison above is the whole argument. Adding £250 a month to this balance cuts years off the term and saves most of the interest, because the extra payment reduces the principal that interest is charged on from the very next month.
Where this goes wrong. The avalanche can start with a large balance that takes a long time to clear, and the lack of early wins is why people abandon it. If motivation is the binding constraint, the snowball's extra cost may be money well spent.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
Typically a few hundred pounds on a mixed debt load of £15,000–£20,000, and more when the rate spread between debts is wide. On similar rates the two methods produce almost identical results.
Usually yes, if you qualify. Moving a 27.9% balance to 0% for 24 months at a 3% transfer fee is cheaper than almost any repayment strategy — provided the balance is actually cleared before the promotional rate ends.
It returns months to clear with the extra. With 6,400 highest-rate debt balance, 27.9 % APR its interest rate and 160 its minimum payment, that comes to 19.6 months. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.