How do you calculate service parts days of supply?

Divide on-hand parts by average daily usage for unprotected days, then divide by your safety factor for protected days. With 1,850 parts, 42 used per day, and a 1.25x factor, that's 44.05 unprotected days falling to 35.24 protected days.

What is a good service parts days of supply?

It depends on criticality and replenishment lead time: fast-moving consumables might run 15-30 days, while critical, long-lead parts are often held at 45-90 days. The protected figure should comfortably exceed your supplier lead time.

What does the safety factor do?

It discounts raw cover to account for demand variability and lead-time risk. A 1.25x factor turns 44.05 raw days into 35.24 protected days - effectively reserving about 20% of cover as a buffer against spikes and late replenishment.

Days of supply vs inventory turns for service parts?

Days of supply is intuitive for stocking a specific SKU and ties directly to lead time and SLAs; turns measures portfolio efficiency over a year. Planners stocking critical spares should lead with days of supply.

Why is protected days lower than unprotected days?

Protected days applies the safety factor as a divisor, so it deliberately reports a more conservative cover. The 1.25x factor in the example cuts 44.05 unprotected days to 35.24 protected days, the number you should plan against.

How do I pick a safety factor?

Base it on demand variability and lead-time reliability for that part. A stable, short-lead consumable might warrant 1.1x; a volatile, long-lead critical spare could justify 1.5x or more to protect uptime.

Aftermarket, Field Service & Service Parts calculator

Service Parts Days of Supply Calculator

Service Parts Days of Supply tells a field-service or aftermarket operation how many days its spare-parts stock will last, before and after applying a buffer for demand and lead-time risk. Service parts planners, depot managers, and field-service supply chains use it to keep critical components on the shelf without drowning slow-moving SKUs in carrying cost. In aftermarket, a single missing part can idle a machine and breach an SLA, so protected days of supply is the metric that links inventory levels to uptime commitments. This calculator discounts raw cover by a safety factor so planners see the realistic, risk-adjusted number rather than an optimistic one.

What this calculator does

Calculate protected days of supply for service parts from inventory on hand, daily usage, and safety-stock factor.
a parts inventory analyst needs to understand how long current service stock will support demand
It divides on-hand service parts by average daily usage to get unprotected days of supply, then divides by a safety factor to give risk-adjusted protected days.

Formula used

Unprotected days of supply = on-hand service parts inventory ÷ average daily usage
Protected days of supply = unprotected days of supply ÷ safety factor

Inputs explained

Service parts inventory on hand: undefined
Average service parts usage: undefined
Demand and lead-time safety factor: undefined

How to use the result

Use it when reviewing spares stocking levels at a depot or van, setting reorder triggers for critical parts, or stress-testing whether current inventory supports an uptime SLA.
It uses a single average daily usage, so for intermittent or lumpy service-part demand - which is common for low-failure-rate components - it can badly misstate real cover; pair it with a demand-pattern review.

Common questions

How do you calculate service parts days of supply? Divide on-hand parts by average daily usage for unprotected days, then divide by your safety factor for protected days. With 1,850 parts, 42 used per day, and a 1.25x factor, that's 44.05 unprotected days falling to 35.24 protected days.
What is a good service parts days of supply? It depends on criticality and replenishment lead time: fast-moving consumables might run 15-30 days, while critical, long-lead parts are often held at 45-90 days. The protected figure should comfortably exceed your supplier lead time.
What does the safety factor do? It discounts raw cover to account for demand variability and lead-time risk. A 1.25x factor turns 44.05 raw days into 35.24 protected days - effectively reserving about 20% of cover as a buffer against spikes and late replenishment.
Days of supply vs inventory turns for service parts? Days of supply is intuitive for stocking a specific SKU and ties directly to lead time and SLAs; turns measures portfolio efficiency over a year. Planners stocking critical spares should lead with days of supply.
Why is protected days lower than unprotected days? Protected days applies the safety factor as a divisor, so it deliberately reports a more conservative cover. The 1.25x factor in the example cuts 44.05 unprotected days to 35.24 protected days, the number you should plan against.

Last reviewed 2026-05-12.