What is days of supply?

Days of supply measures how many days your current inventory will last based on average daily sales. Formula: Inventory on Hand / Average Daily Units Sold.

What is a good days of supply for ecommerce?

For many ecommerce brands, 30 to 90 days is a common planning range, but the right target depends on lead times, seasonality, and service-level goals.

How do I calculate how long my stock will last?

Divide your current sellable inventory by average daily unit sales. If you have 540 units and sell 18 per day, your stock will last 30 days.

What is the difference between days of supply and inventory turnover?

Days of supply expresses inventory as time remaining, while inventory turnover shows how many times inventory is sold and replaced over a period. They are related but used differently.

Should I include safety stock in days of supply?

You should track both total days of supply and usable days of supply above safety stock. The second number is more useful for replenishment timing.

How do I calculate days of supply in Shopify?

Export units sold from Shopify Analytics, calculate average daily demand, then divide available inventory by that daily rate. Shopify does not calculate days of supply natively for most merchants.

Days of Supply Calculator for Shopify Brands

Definition: Days of supply measures how many days your current stock on hand will last based on average daily demand.

Days of supply answers one of the most practical inventory questions in ecommerce: if I do not reorder today, how long until I run out? It is one of the simplest but most useful planning metrics because it translates stock into time. Merchants use it to spot stockout risk, overstock, and whether they have enough coverage to make it through supplier lead times and seasonal peaks.

Days of Supply Calculator formulas

Basic days of supply

Use when: Use for day-to-day replenishment planning when you want a fast answer to how long current stock will last.

Days of Supply = Inventory on Hand / Average Daily Units Sold

Days of Supply = Inventory on Hand divided by Average Daily Units Sold

Symbol	Variable	Description
On Hand	Inventory on hand(units)	Sellable units currently available for a SKU or product group.
Davg	Average daily demand(units / day)	Average number of units sold per day over your chosen lookback period.

Limitation: Assumes future demand will be similar to recent average demand. Can understate risk during promotions or overstate risk after temporary sales spikes.

Usable days of supply above safety stock

Use when: Use when you want to know how many days remain before you hit your buffer threshold rather than absolute zero.

Usable DOS = (Inventory on Hand - Safety Stock) / Average Daily Units Sold

Usable Days of Supply = Inventory on Hand minus Safety Stock, divided by Average Daily Units Sold

Symbol	Variable	Description
On Hand	Inventory on hand(units)	Current sellable stock.
SS	Safety stock(units)	Minimum inventory buffer you do not want to consume unless demand or lead time deviates from plan.
Davg	Average daily demand(units / day)	Average daily unit sales.

Limitation: Requires a sensible safety stock number. If safety stock is guessed rather than calculated, the output can mislead.

Months of inventory

Use when: Use for financial planning, cash planning, and supplier discussions where inventory is often discussed in months rather than days.

Months of Inventory = Days of Supply / 30.4

Months of Inventory = Days of Supply divided by 30.4

Symbol	Variable	Description
DOS	Days of supply(days)	Result from the core days-of-supply formula.

Limitation: This is a presentation format, not a separate operational formula. Days are usually better for short-term replenishment decisions.

Step-by-step examples

Scenario: “A Shopify supplements brand has 540 units on hand and sells 18 units per day on average.”

Inventory on hand: 540 units

Average daily demand: 18 units / day

1Days of Supply = 540 / 18
2Days of Supply = 30 days

30 days of supply

Interpretation: If demand stays stable, this SKU will last about 30 days before stock reaches zero. If supplier lead time is 21 days, the brand still has a narrow planning window.

Scenario: “A cosmetics SKU has 1,200 units on hand, safety stock of 300 units, and average daily demand of 25 units.”

Inventory on hand: 1,200 units

Safety stock: 300 units

Average daily demand: 25 units / day

1Usable inventory = 1,200 - 300 = 900 units
2Usable DOS = 900 / 25
3Usable DOS = 36 days

36 usable days of supply above safety stock

Interpretation: This merchant has 36 days before dipping into buffer inventory. That is the more useful trigger point for replenishment planning.

Typical days-of-supply reference points

Category	Benchmark	Note
Fast-moving replenishable SKUs	14–45 days	Common for consumables and short lead-time products.
General ecommerce target	30–90 days	Broad planning range, depending on lead time and demand volatility.
Long lead-time imported SKUs	60–120+ days	Often required when production plus freight cycles are long.
Under 14 days	High stockout risk	Usually too low unless lead times are extremely short and reliable.
Over 120 days	Potential overstock	Review demand, seasonality, and markdown risk.

A good days-of-supply target depends on supplier lead time, service-level goals, and how seasonal the SKU is. The number should not be judged in isolation.

Critical pitfalls to avoid

Using revenue instead of units sold

Days of supply should be based on unit velocity, not revenue. A high-priced SKU can distort the result if you use sales value.

Fix: Use average daily units sold per SKU or per variant.

Ignoring seasonality

Using a 90-day average in April to predict Black Friday inventory coverage will understate demand risk badly.

Fix: Use a seasonally relevant lookback period or a peak-season adjusted demand forecast.

Counting unsellable inventory

Damaged, reserved, or dead stock units inflate on-hand inventory and make days of supply look healthier than it really is.

Fix: Use sellable available inventory only.

Using zero-sales periods incorrectly

A long flat period for a dormant SKU can artificially increase days of supply into meaningless territory.

Fix: Review slow movers separately and pair DOS with sell-through or dead-stock logic.

Ignoring safety stock

A SKU may show 40 days of supply, but if 15 days of that is safety stock, the merchant actually has much less usable coverage.

Fix: Track both total DOS and usable DOS above safety stock.

Shopify-specific tips

Pull units sold from Shopify Analytics using a consistent date range, then divide by the number of days in that range to get average daily demand.
Use available inventory rather than total inventory if some units are committed, reserved, or unsellable.
For multi-location merchants, calculate days of supply per location first. Aggregated stock can hide local stockout risk.
For seasonal products, compare against the equivalent period last year rather than a generic trailing average.
Days of supply is most useful when paired with reorder point and safety stock, not used as a standalone replenishment rule.

Days of Supply Calculator