> For the complete documentation index, see [llms.txt](https://docs.prophetmarket.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.prophetmarket.ai/core-concepts/prices-spread-and-rounding.md).

# Prices spread and rounding

*How the quoted price is built, what it costs you, and why rounding always favours you.*

Every market has a midpoint, which is the fair estimate of probability, and a spread, which is the gap between what you pay to buy and what you receive to sell. The spread is inside the prices you are quoted.

## Buying and selling are different prices

**Worked example**

```
Buy YES   →  you pay 65¢ per share
Sell YES  →  you receive 63¢ per share
```

The gap between those two numbers is the cost of trading.

The comparison many people make is a currency exchange booth. The board shows one rate to buy and a slightly worse rate to sell, and the booth earns the difference. Prophet works the same way, and the practical implication is the same: a round trip costs you the spread twice, so the shorter your holding period, the more the spread matters relative to your view.

{% hint style="danger" %}
**The spread is substantial and it varies**

Prophet's spread is meaningful, currently around twenty percent, and the rate can be adjusted depending on the risk profile of a market. Always check the buy and sell prices for a market before sizing a position, and treat short term in and out trading as expensive.
{% endhint %}

## Prices are probabilities

| Quoted YES price | Implied probability | What it says                            |
| ---------------- | ------------------- | --------------------------------------- |
| 5¢               | 5%                  | The market considers this very unlikely |
| 27¢              | 27%                 | Unlikely, but far from dismissed        |
| 50¢              | 50%                 | Genuinely uncertain                     |
| 82¢              | 82%                 | Likely, with real residual doubt        |
| 94¢              | 94%                 | Close to settled                        |

Reading a price as a probability is the most useful habit you can build. It reframes the decision from whether an event will happen to whether the quoted number is too high or too low, which is a much more tractable question.

## Rounding is conservative toward you

Prices have to be rounded to whole cents, and Prophet rounds deliberately in the direction that protects the trader rather than the platform. Ask prices round up and bid prices round down, which means the number on the screen is the worst case you will experience, not an idealised midpoint you then fail to get.

**If the screen says buy at 65¢**

You will never be charged more than 65¢ per share. You may pay exactly that.

**If the screen says sell at 63¢**

You will never receive less than 63¢ per share. You may receive exactly that.

{% hint style="success" %}
**The price you see is the price you get**

Because rounding runs in your favour, there is no hidden slippage between the displayed quote and the executed price. What you see is a commitment, not an estimate.
{% endhint %}


---

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