How do you calculate days of supply?

Divide inventory on hand by average daily usage. With 4,500 units on hand and 300 units/day of usage that is 15 unprotected days. Dividing again by a 1.2 safety multiplier gives 12.5 protected days.

What is the difference between protected and unprotected days?

Unprotected days (15 here) is the raw inventory-divided-by-usage figure. Protected days (12.5) derates that by the safety multiplier so demand or supply variability does not eat into your true buffer.

What is a good number of days of supply?

It depends on replenishment lead time. A healthy target is DOS comfortably above your supplier lead time plus review period — if your part has a 10-day lead time, 12.5 protected days is tight but workable; 5 days would be a stockout risk.

What does the safety factor do in this calculation?

The safety factor (1.2 in the example) shrinks reported coverage to build in a margin. A 1.2x factor treats 15 nominal days as only 12.5 reliable days, reserving roughly 17% of stock as buffer against variability.

Days of supply vs inventory turns — what's the relationship?

They are inverses scaled to a period. Days of supply is roughly 365 divided by annual inventory turns. Twelve days of supply implies turns of about 30 per year, which is very lean.

Should I use average or peak daily usage?

Use average usage for routine planning, but stress-test critical parts with peak usage. If your peak day is 400 units instead of 300, your 12.5 protected days collapses to under 9 — that gap is exactly what the safety factor is meant to absorb.

Supply Chain & Procurement calculator

Days of Supply Calculator

Days of Supply (DOS) tells you how many days your current inventory will last at a given consumption rate before you run out. Materials planners, buyers, and plant inventory analysts use it to decide when to reorder, how much safety stock to hold, and which SKUs are at risk of starving the line. It is the single most intuitive way to translate a raw on-hand quantity into a time horizon a planning meeting can act on. Applying a safety multiplier converts the naive coverage figure into a more conservative 'protected' number that absorbs demand spikes and supplier variability.

What this calculator does

Calculate days of supply from inventory, daily usage, and policy factor.
Use it when days of supply in supply chain and procurement is being sized for a buffer or safety stock review.
It computes how many days of demand your on-hand inventory covers, both at face value and after derating by a safety multiplier.

Formula used

Protected days = inventory on hand ÷ daily usage ÷ safety multiplier

Inputs explained

Current inventory on hand: Units in stock right now.
Average daily demand: Average units consumed or shipped per day.
Coverage safety factor: Multiplier for desired buffer (1.0 = none).

How to use the result

Use it during reorder reviews, MRP exception triage, or when comparing coverage across components feeding the same line.
It assumes a flat, constant daily usage rate, so for seasonal or lumpy demand the protected days can mislead — average usage hides the peaks that actually cause stockouts.

Current U.S. benchmarks

U.S. manufacturing runs at 75.6% of capacity (Federal Reserve, May 2026). New factory orders are up 2.3% year over year (Census).
Sourcing currencies as of 2026-07-02 (Federal Reserve H.10): 6.7886 CNY and 17.4524 MXN per USD. Landed-cost comparisons move with these daily rates.
U.S. iron and steel imports ran $2.1B in May 2026 (Census International Trade). The U.S. ran a trade deficit of $0.4B in the category that month. Import volumes are the pressure gauge behind tariff and reshoring decisions.

Common questions

How do you calculate days of supply? Divide inventory on hand by average daily usage. With 4,500 units on hand and 300 units/day of usage that is 15 unprotected days. Dividing again by a 1.2 safety multiplier gives 12.5 protected days.
What is the difference between protected and unprotected days? Unprotected days (15 here) is the raw inventory-divided-by-usage figure. Protected days (12.5) derates that by the safety multiplier so demand or supply variability does not eat into your true buffer.
What is a good number of days of supply? It depends on replenishment lead time. A healthy target is DOS comfortably above your supplier lead time plus review period — if your part has a 10-day lead time, 12.5 protected days is tight but workable; 5 days would be a stockout risk.
What does the safety factor do in this calculation? The safety factor (1.2 in the example) shrinks reported coverage to build in a margin. A 1.2x factor treats 15 nominal days as only 12.5 reliable days, reserving roughly 17% of stock as buffer against variability.
Days of supply vs inventory turns — what's the relationship? They are inverses scaled to a period. Days of supply is roughly 365 divided by annual inventory turns. Twelve days of supply implies turns of about 30 per year, which is very lean.

Last reviewed 2026-05-12.