How do you calculate days of supply?

Divide inventory on hand by average daily demand. With 4,500 units on hand and 300 units/day of demand that is 15 days of supply - the canonical DOS. Dividing by the 1.2 safety multiplier gives a conservative 12.5-day planning figure.

What is the difference between cycle stock and the required buffer?

Days of supply (15 here) is the standard inventory-divided-by-demand metric - the number to quote and benchmark. The conservative coverage row (12.5) derates it by the safety multiplier for planning, so 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; with a 10-day lead time, 15 days of supply (12.5 conservative) is tight but workable, while 5 days would be a stockout risk.

What does the safety factor do in this calculation?

The safety factor (1.2 in the example) produces the conservative secondary figure. It treats the 15-day DOS as only 12.5 reliable days for planning, reserving roughly 17% of stock as buffer against variability - the headline stays the standard metric.

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 demand for routine planning, but stress-test critical parts with peak demand. If your peak day is 400 units instead of 300, the 15-day DOS collapses to 11.25, 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

Days of supply = inventory on hand ÷ average daily demand
Conservative coverage = days of supply ÷ safety factor

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 75.7% of capacity (Federal Reserve, Aug 2026). New factory orders are up 8.5% year over year (Census).
USITC reported an average effective tariff statistic — a calculated-duty rate of 12.4% of covered customs value in 2025 across the 57 manufacturing import families MFG Calcs tracks, up from 3.3% the year before. This is a statistical aggregate, not verified cash paid or an entry-specific legal rate. Statutory and effective rates by family are at mfgcalcs.com/tariffs.
Sourcing currencies as of 2026-10-02 (Federal Reserve H.10): 6.7038 CNY and 18.192 MXN per USD. Landed-cost comparisons move with these daily rates.

Common questions

How do you calculate days of supply? Divide inventory on hand by average daily demand. With 4,500 units on hand and 300 units/day of demand that is 15 days of supply - the canonical DOS. Dividing by the 1.2 safety multiplier gives a conservative 12.5-day planning figure.
What is the difference between cycle stock and the required buffer? Days of supply (15 here) is the standard inventory-divided-by-demand metric - the number to quote and benchmark. The conservative coverage row (12.5) derates it by the safety multiplier for planning, so 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; with a 10-day lead time, 15 days of supply (12.5 conservative) is tight but workable, while 5 days would be a stockout risk.
What does the safety factor do in this calculation? The safety factor (1.2 in the example) produces the conservative secondary figure. It treats the 15-day DOS as only 12.5 reliable days for planning, reserving roughly 17% of stock as buffer against variability - the headline stays the standard metric.
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 demand for routine planning, but stress-test critical parts with peak demand. If your peak day is 400 units instead of 300, the 15-day DOS collapses to 11.25, that gap is exactly what the safety factor is meant to absorb.

Related guides

Last reviewed 2026-08-13.