Fill rate vs service level: formulas and the difference

Fill rate and service level both describe how well you keep items in stock, but they measure different things, and the same inventory can score very differently on each. Here is how to calculate both and when to use which.

Fill rate

Fill rate is the share of demand you ship from stock. It comes in three versions, and each one is harsher than the last:

Type	Calculation	Example	Result
Unit fill rate	Units shipped ÷ units ordered	940 ÷ 1,000	94%
Line fill rate	Complete lines ÷ total lines	180 ÷ 200	90%
Order fill rate	Complete orders ÷ total orders	82 ÷ 100	82%

One missing item can spoil a whole order, which is why the order figure is the lowest. Always say which version you mean.

Cycle service level

Cycle service level is the chance of getting through a replenishment cycle without running out.

Cycle service level = cycles with no stock-out ÷ total cycles

If an item was replenished 20 times and ran out in 2 of those cycles, its cycle service level is 18 ÷ 20, or 90%.

The difference

	Fill rate	Cycle service level
Question it answers	How much demand did we meet?	How often did we avoid running out?
Counts	Units, lines or orders	Replenishment cycles
Effect of a small shortage	Lowers it a little	Counts as a full miss
Best used for	Reporting to customers and managers	Setting safety stock

Because a shortage of two units counts as a whole missed cycle, fill rate is usually the higher number. An item can have a 98% fill rate and a 90% cycle service level if its shortages are frequent but small.

Which one to use

Use fill rate to report performance, since it is closer to what customers feel. Use cycle service level to set stock, because the Z-value in the safety stock formula comes from it. Set different targets by item class, so your safety stock goes where a miss costs most. See ABC-XYZ classification.

Traps to avoid

• Backorders. Decide whether a late shipment counts as filled, and be consistent.
• Mixed definitions. Sales and operations may each be quoting a different version.
• Averages. A good company-wide number can hide poor service on your best sellers.

To raise fill rate without simply adding stock, read how to prevent stockouts or our guide to inventory planning software.

Frequently asked questions

How do you calculate fill rate?

Divide what you shipped from stock by what was ordered. You can measure it in units, order lines or whole orders.

Is fill rate the same as service level?

No. Fill rate measures how much demand was met. Cycle service level measures how often you avoided running out during a replenishment cycle.

Why is fill rate usually higher than service level?

A small shortage only reduces fill rate slightly, but it counts as a full miss for cycle service level.