Methodology

How our calculators work

Betting calculators are simple to build and surprisingly easy to get wrong. A single rounding error changes a payout. This page explains exactly how slipmath calculates, and how we check it.

1. Exact arithmetic

Computers normally store numbers like 1.909 approximately. We don’t. Every calculation runs on exact fractions: −110 becomes exactly 21/11, never 1.90909…. Results are rounded only once, at the very end, for display.

2. Documented rounding

• Money — 2 decimal places, halves rounded away from zero. A $100 bet at −110 returns $190.91.
• Decimal odds — 2 decimal places.
• American odds — whole numbers, halves away from zero. Fractional odds of 40/173 are exactly −432.5 American, shown as -433.
• Fractional odds — the exact fraction when its denominator is 100 or less; otherwise the closest fraction with a denominator up to 100.
• Probabilities — percentages with 2 decimal places.

Some bookmakers round payouts down or cap maximum winnings, so their figure can differ by a cent or more. Always check your bet slip.

3. Rules we apply

• Parlays: a pushed or void leg counts as 1.00 and the parlay continues; any losing leg means no payout.
• No-vig / fair odds: proportional method — each implied probability divided by the market total.
• Free bets: stake-not-returned pays stake × (odds − 1); stake-returned pays stake × odds. Lay stakes use the standard exchange formula with commission.

4. How we test

• A written specification defines every formula, rounding rule and error case before any code is written.
• An independent reference calculator generates 198 test cases, including published examples (for instance a three-leg −110 parlay) and edge cases.
• The calculation engine is written twice, independently. Both versions must match the reference on every case and agree with each other on thousands of random inputs. This process caught real rounding errors before launch.
• Property tests check rules that must always hold — for example, that fair probabilities always add up to 100%.

5. Corrections

If you think a result is wrong, tell us with the inputs you used. Confirmed errors are fixed, added as a permanent test case, and noted on this page.

Maintained by the slipmath Editorial Team. Last reviewed 2026-10-07.