Enter a total stake and the prices you can get, and this splits the stake so the return is identical whichever selection wins. It also shows the book percentage, so you can see when the split is an arb.
Dutching means backing several selections in the same event so that you win the same amount whichever one lands. Each stake is set in proportion to one over its price, then scaled so the parts add up to your total stake. Put $100 across 2.10, 3.40 and 5.50 and the split is roughly $50.01, $30.89 and $19.10, returning about $105.03 whichever wins.
The key number is the book percentage — the sum of one over each price. Under 100% and the split is an arbitrage that profits whatever happens; over 100% and you are paying the bookmaker's margin, so dutching only makes sense when you have narrowed the field to selections you genuinely fancy.
You do not have to take every price at one book. Taking the best available price for each selection across different bookmakers lowers the book percentage and is usually where any value lives — the maths does not care which book each price comes from.