Volatility-scaled position size: same risk, different lots

[Amanhasnoname]

Using a fixed lot size means risk varies with volatility. On a quiet day, a 5 pip stop is relatively wide. On a wild day, it's tight and gets hit constantly.

Scaling position size to volatility keeps risk consistent. Measure the current average range, set the stop as a multiple of it, and compute the lot size that risks a fixed amount over that stop distance.

In quiet conditions, stops tighten and lots grow. In fast conditions, stops widen and lots shrink. The dollar risk per trade stays the same.

It also protects against the worst surprises. Position sizes automatically fall in volatile moments, which are also the moments of poor execution.

Complexity is modest. Many position-size calculators accept an ATR input. For automated systems, it's a few lines of code.

One warning: higher lots in calm periods should come with a check on spread, since a tiny stop with a normal spread has a bad cost ratio.

A refinement is to cap the maximum lot size regardless of the formula. When volatility collapses to almost nothing, the calculation produces huge positions with tiny stops, and any spread spike or slippage then turns into an outsized loss.

Do you adjust size for volatility, or keep it fixed?