An accumulator multiplies prices, which is why everyone likes them. What gets less attention is that it multiplies margins too: each leg carries the book's overround, and the overrounds compound exactly like the prices do. Three legs at 4.7% each do not hold 4.7%; they hold about 14.8%. Here is the math, worked.
The multiplication nobody reads
Take three matches, each priced 1.91 on both sides. Each market's implied probabilities sum to 104.71%, an overround of 4.71%. The treble pays 1.91 cubed, which is 6.97.
Now remove the margin per leg with the no-vig calculator: each 1.91 pair normalizes to a fair 2.00, so the fair treble pays 2.00 cubed, 8.00. The book pays 6.97 against a fair 8.00, because 1.0471 cubed is 1.148. The combined market carries 14.8%, and the treble returns about 87 cents on the fair unit, checked against the calculator's proportional rule on 2026-10-12.
That is the whole mechanic: the margin on an acca is the product of the margins on its legs.
Why accumulators feel better than they are
The posted price of a treble looks generous because multiplication makes big numbers. 6.97 feels like a big win from three even-money picks. But the book's take grew just as fast: from 4.7% per leg to 14.8% on the ticket. The bigger the acca, the larger the share of the pool the book keeps, which is exactly why books promote them.
A four-leg version of the same matches holds about 19.5%, and a six-leg one about 31%. The curve only bends one way.
Computing the fair parlay correctly
The order is fixed: remove the margin on each leg first, then multiply the fair probabilities. Proportional removal per leg, then the product. Removing the margin after combining, or multiplying raw implied probabilities, bakes the legs' margins into your fair price and overstates it.
The calculator handles one market at a time, so the workflow is per leg: fair chance per leg, then multiply across legs, then take 1 over the product for the fair parlay price.
The other side of the coin
Edges compound too. If you genuinely hold a +3.8% edge per leg, like the worked example on this site's sister calculator pages, three such legs compound to about +11.5% on the treble. An acca is a multiplier with no opinion: it multiplies whatever you bring, margin or edge. The discipline is the same one the whole site keeps: know the fair price first, then decide.
Prices multiply, and so do margins. Compute the fair product per leg, and the acca stops looking free.