Use a named reference price
The spread is the difference between the best bid and best ask. The midpoint is halfway between them. For a buyer, the available ask is more relevant to immediate execution than the midpoint; for a seller, the available bid matters. State which reference you use whenever you report slippage.
In this fictional snapshot, the best bid is $0.41 and the best ask is $0.45, so the spread is $0.04 and the midpoint is $0.43. None of those numbers guarantees that 12 shares are available at one price. These figures are an arithmetic exercise, not a current market quote.
Walk the available quantity, level by level
The weighted average is $5.62 ÷ 12 = approximately $0.4683 per share, before separate fees. Multiplying the best ask by 12 gives $5.40 and understates this modeled cost by $0.22. A midpoint-based estimate of $5.16 understates it by $0.46.
Of that $0.46 midpoint difference, $0.24 comes from crossing from $0.43 to the initial $0.45 ask for 12 shares; $0.22 comes from consuming higher levels. This breakdown describes this snapshot only. Quotes may disappear, and another trader may consume them before your order arrives.
| Ask per share | Shares available | Shares used in example | Notional used |
|---|---|---|---|
| $0.45 | 4 | 4 | $1.80 |
| $0.47 | 6 | 6 | $2.82 |
| $0.50 | 10 | 2 | $1.00 |
| Total | 20 | 12 | $5.62 |
A price limit and a total budget answer different questions
A buy limit of $0.47 would exclude the $0.50 level. Under the unchanged example book, only ten shares are offered within that limit. Whether the remainder rests or is canceled depends on the order instruction. You cannot infer a 12-share fill from the average price being below $0.47: the final two shares individually cost $0.50.
Now suppose an offline test uses a $5.70 total budget and reserves $0.20 for possible additional costs. That reserve is a chosen testing buffer, not a Polymarket fee rate. The modeled $5.62 notional plus $0.20 reserve is $5.82, so the test should block that size. A total budget check must include the costs your integration actually pays.
Verify fees for this market and this execution
Current Polymarket documentation distinguishes maker and taker treatment and describes fees on certain markets. Do not rely on an old statement that every market is free, or treat a category coefficient as a flat percentage of the order value. Read the current market parameters and fee documentation before estimating, then reconcile the actual records afterward.
A limit order that immediately consumes an existing quote can be a taker order. An order may also have fills at multiple prices. Separately identify any routing or builder charge, wallet or network cost, and funding-provider charge that applies to the service you actually use. Do not assume a fee charged by one provider is a SharkRule fee.
Avoid counting the spread twice
Actual execution notional already reflects the prices you paid. If the 12-share example executes for $5.62, do not add another $0.46 as a fee: that difference only compares the execution with the $5.16 midpoint benchmark. Additional separately charged fees belong on a different line.
For a later sale, read the bid side and its quantities again. Reusing the earlier ask ladder is wrong. There is no guarantee of an immediately available exit, and deposited dollars, collateral units and outcome shares should not be mixed in one column. Keep every amount labeled with its unit and whether it is estimated or confirmed.
A cost record that can explain a discrepancy
For limit orders resting in a queue, a visible snapshot alone cannot prove when your order would fill. The worksheet is for organizing a quote or receipt, not forecasting returns. A positive difference from one benchmark does not establish that a trade was suitable or profitable.
- Save the outcome token, snapshot time and named reference price.
- Record each assumed or confirmed fill quantity and price.
- Calculate total notional and weighted average from those rows.
- Add only costs charged separately; retain the source and observation time for each estimate.
- Compare the result with both the per-share limit and the total spending cap.
- If actual data is missing, label the cost incomplete instead of replacing it with zero.
Custody, permissions and eligibility
Non-custodial means funds remain in a user-controlled wallet; it does not remove trading permissions, smart-contract risk or the possibility of loss. SharkRule is independent from Polymarket, with no affiliation or endorsement implied. This is software education, not financial or investment advice. Trading eligibility follows current Polymarket geographic restrictions.
Check current geographic restrictionsFrequently asked questions
Is slippage always measured against the midpoint?
No. It can be measured against a quote, decision price or another stated benchmark. Name the benchmark; otherwise two slippage figures may not be comparable.
Are all limit orders fee-free maker orders?
No. A marketable limit can consume resting liquidity. Verify the actual maker/taker role and current market fee parameters.
Can I use the example as a live quote?
No. The depth and cost buffer are invented for calculation practice. Obtain fresh, correctly identified market data for any real evaluation.
Your working worksheet
Save a copy and fill in the fields locally. Use public identifiers only; never include passwords, private keys, seed phrases or API secrets. This is a planning document, not an account connection or trading tool.
Download the worksheet (.txt)Sources and further reading
How this guide was prepared
Prepared with AI assistance using the official sources linked above. The numerical examples and decision boundaries were checked with local, deterministic test fixtures. No live trades, performance backtest or independent expert review was conducted. Documentation was checked on 12 September 2026; check the linked provider guidance for later changes.