Your assumptions
Illustrative scenario · no live market data
Simulation result
Expected net result = shares × probability − (shares × price + costs).
Assumptions: binary outcome, $1 payout per winning share, position held to resolution, all costs entered as a fixed total. No fees, slippage or probabilities are fetched from a platform.
How the arithmetic works
If you buy n shares at price c per share, pay a nonnegative total fixed cost F, and treat p as your own assumed probability of YES, then your up-front outlay is n*c + F. A winning position returns n*(1 - c) - F, a losing position returns -n*c - F, and the average over those two hypothetical outcomes is n*(p - c) - F. The break-even probability is c + F/n. Every value you type stays in your browser: these figures do not place an order, connect a wallet, or get sent to a server.
A worked run shows the shape of a result. With n = 100 shares, c = 0.40, p = 0.60, and F = 2, the outlay is 42, the win branch 58, the loss branch -42, and the expected net 18. The break-even probability is 42%. That 18 is an average across hypothetical outcomes, not the result this particular position will produce. The actual position ends in one of the two branches, so plan for the -42 loss case as a real possibility, not just the favorable average.
Reading the output and comparing positions
The expected net is only as good as the p you supply, and p is a subjective assumption rather than a measured fact. For a separate record of resolved forecasts, open the Brier score calculator. No arithmetic here recommends a probability, an order size or a portfolio weight.
The formula assumes one ordinary contract, one execution price, fixed nonnegative total costs, and full settlement through resolution. If your execution price changes between fills, fees vary with size or timing, or settlement is disputed or nonstandard, the numbers fall outside this simplified model and the result should be treated as a rough check, not an exact projection.
What this tool is and what it does not cover
This is an educational calculator, not investment advice, and it does not execute trades. It computes only the arithmetic in the formulas above from the numbers you enter. Buying or selling prediction-market contracts involves additional risks the arithmetic does not capture, including platform availability or access restrictions in your location and the possibility that withdrawals or market access are limited by local rules. Availability and local rules vary, and this page does not claim that any particular jurisdiction permits trading; you should check the rules that apply to you before relying on any output here.
PolyZeno is an independent educational publication initiated by Meydeey and is not an official Polymarket service. Writing and automated fact and language review use DeepSeek V4.1 Flash; AI review is a checking step, not proof, and no human expert biographies or track records are claimed. There are currently no affiliate links or active commercial partnerships on this site. Future commercial links would require verified terms, eligibility, and clear disclosure, and commissions would not determine editorial conclusions. The calculator runs on visitor-entered values locally and does not send them to a server.
PolyZeno. Automated review with DeepSeek V4.1 Flash.