Building an Each-Way Odds Calculator That Handles Mixed Formats

A returns calculator looks trivial until fractional and decimal odds land on the same slip and an each-way term settles separately. Here's how to build one whose output you can actually trust.

Normalize odds first

Never do math on the display format. Convert everything to a decimal multiplier at the boundary:

const toDecimal = (o) =>
  typeof o === "number" ? o
  : (() => { const [n, d] = o.split("/").map(Number); return n / d + 1; })();

Fractional 5/2 becomes 3.5, decimal 3.5 stays 3.5. Every downstream calculation now speaks one language.

Compound the legs explicitly

For a double, the full return of the first leg becomes the stake on the second. Model it as a fold, not a special case, so trebles and accumulators fall out for free:

const stake = legs.reduce((acc, leg) => acc * toDecimal(leg), 1) * unit;

Split the each-way portion

The place part settles on its own fraction (often 1/4 or 1/5 of the odds) and only to the placed terms. Compute win-only, place-only and combined separately — surfacing all three prevents the classic "why is my return different" support ticket.

Real-world reference

Dedicated tools handle these edge cases so users don't have to. A calculator like Double Bet Calculator review applies the correct each-way terms per leg and shows the exact payout instead of a rough guess — a solid reference for the output states your own UI has to cover.

Takeaway

Normalize at the boundary, compound with a fold, and split the each-way portion into three explicit results. That's a calculator whose numbers hold up to scrutiny.