How do you calculate return rate and return delay?

Return rate and return delay are both calculated from historical order data by matching each returned unit back to the original sales order it came from.

Return rate is the share of units sold, over a defined lookback window, that were eventually returned, calculated at whatever granularity (SKU, product family, or channel) the variance in your catalogue actually requires; a single company-wide average tends to be wrong for almost every individual product it's applied to.

Return delay is the average time between the sale date and the return date for that same matched population, and it should be split by sales channel wherever return policies differ, since a return window that runs two weeks through one channel and several months through another produces very different inbound timing for what looks like the same product.

Once both figures are calculated, they're applied to the constrained outflow (the smaller of available stock and forecasted demand, not the forecast alone) to produce a day-by-day or week-by-week returns forecast.