Days of Supply: Formula and How to Use It

Days of supply is the number of days the inventory on hand would last at the expected rate of demand, and it is one of the most practical measures in planning because it translates a pile of stock into a time horizon that everyone understands. A buyer who hears "we have 14 days of supply and our lead time is 30 days" knows immediately that there is a problem. A finance director who hears "we have 180 days of supply on a slow item" knows that cash is stuck. This guide explains the formula, shows the difference between days of supply and the backward-looking measure days inventory outstanding, works through a simple example and a time-phased example with checked arithmetic, shows how to set targets and quantify excess in dollars, and explains where the simple method breaks down. It is part of demand planning and inventory optimization, follows inventory turnover ratio formula and inventory turnover, and leads to safety stock formula and reorder point formula. The cash effect connects to days inventory outstanding and cash flow management.

The formula

Days of supply = inventory on hand (in units) ÷ average daily demand (in units).

The numerator can be on hand only, or on hand plus incoming orders, depending on the question. The denominator should be the expected demand from the forecast, not just last month's average, particularly when demand is changing. The result is in days of coverage. Using value instead of units, days of supply equals inventory at cost ÷ forecast cost of goods sold per day, which gives the same answer when the unit cost is constant.

Forward-looking versus backward-looking

Days inventory outstanding uses historical cost of goods sold and average inventory, so it tells you how long stock has taken to sell. Days of supply uses forecast demand and current inventory, so it tells you how long stock will last. The two can diverge sharply when demand is changing. A company with falling demand may have a healthy backward-looking figure and an alarming forward-looking one. Planners need the forward-looking view; finance reporting usually uses the backward-looking one. See days inventory outstanding.

A simple example

An item has 9,000 units on hand and a forecast of 4,500 units over the next 30 days, which is 150 a day (illustrative). Days of supply = 9,000 / 150 = 60 days. If 3,000 more units are on order and arrive within the period, the days of supply including incoming stock is (9,000 + 3,000) / 150 = 80 days. Check: 12,000 / 150 = 80. If the lead time is 45 days and the safety stock target is 14 days, the stock position should be about 59 days when the next order is placed, so coverage of 60 is about right today, but 80 including the open order is high and might justify delaying or reducing the next purchase.

Using it to spot problems across items

Compare days of supply with a target for each item, derived from lead time, review cycle and safety stock. Suppose three items (illustrative):

Check: 1,200 / 40 = 30; 5,000 / 20 = 250; 300 / 60 = 5. Items A and C are at risk of stockout, with C in immediate danger. Item B carries far more stock than it needs. The excess on item B in units is 5,000 - (20 × 60) = 5,000 - 1,200 = 3,800 units. At a unit cost of $12, that is 3,800 × 12 = $45,600 of excess stock, which could be addressed by pausing replenishment and, if the item is slow, by a promotion or return to the vendor. A report like this, sorted by gap in dollars, gives planners a prioritized list every week.

A time-phased example

Simple days of supply assumes a constant rate of demand, which is wrong when demand is seasonal or a promotion is coming. A time-phased calculation walks forward through the forecast. Suppose 3,600 units are on hand, and the forecast is 2,000 units over the next 28 days (71.4 a day) and 3,000 units over the following 28 days (107.1 a day). The first 28 days consume 2,000 units, leaving 1,600. At the higher rate of 3,000 / 28 = 107.14 units a day, the remaining 1,600 units last 1,600 / 107.14 = 14.9 days. Total days of supply = 28 + 14.9 = 42.9 days. A simple calculation using the 28-day average would suggest 3,600 / 71.4 = 50.4 days, overstating coverage by about 7 days. When a peak is coming, that difference is the gap between a comfortable position and a stockout. Time-phased supply planning does this automatically for every item. In a spreadsheet, it can be done for a priority list.

Setting target days of supply

A target is built from the components: lead time, review period and safety stock. For an item replenished every two weeks, with a 30-day lead time and a 10-day safety stock, the maximum coverage after receiving an order is roughly review period plus safety stock, 14 + 10 = 24 days, plus a share of the order cycle; and the minimum before reordering is lead time plus safety stock, 40 days of pipeline and on-hand. The sum gives a range in which coverage should sit. Set separate targets by class: higher for A items where service matters, lower for C items. Review targets quarterly as lead times and demand change. See reorder point formula.

Translate days into dollars

To show leadership the effect of days of supply, convert it to cash. If annual cost of goods sold is $30 million, one day of supply is 30,000,000 / 365 = $82,192. A program to reduce average coverage by 8 days releases about 8 × 82,192 = $657,500. Check: 82,192 × 8 = 657,536. Compare with the cost of any service risk, and plan the reduction by item: take it from the items with the greatest excess coverage and protect those with short coverage.

Limits of the method

Days of supply depends on the quality of the demand forecast. A biased forecast makes coverage look better or worse than it is. It assumes that demand continues at the forecast rate and does not capture variability, so two items with the same days of supply may have very different stockout risk. It says nothing about when stock is needed, which matters with long lead times and inbound timing. It can be gamed by changing the forecast. And it needs a sensible rule for items with zero or tiny forecast, where the result is infinite or meaningless; cap it or flag it as no demand. Use days of supply with safety stock logic, not instead of it.

Common mistakes

1. Using last month's sales as daily demand when demand is changing.
2. Mixing on hand and incoming without saying which.
3. A single target for all items.
4. Ignoring seasonality and promotions.
5. Reporting averages that hide extremes.
6. No rule for zero-demand items.

FAQs

What is days of supply?

The number of days current inventory would last at the expected rate of demand: inventory on hand divided by average daily demand.

How is days of supply different from days inventory outstanding?

Days of supply looks forward using forecast demand. Days inventory outstanding looks backward using historical cost of goods sold.

What is a good days of supply?

It depends on lead time, review cycle, variability and service targets. Set a target for each item or class and compare.

Should I include incoming orders?

Report both. On hand shows current coverage, and on hand plus incoming shows projected coverage.

How do I handle seasonal items?

Use a time-phased calculation that walks forward through the forecast, instead of a flat average.