How much should you order, and how often?
Too much stock ties up cash, too little triggers stockouts. Safety stock is the dial that arbitrates between those two risks. Here is the definition, the most widely used calculation formula, a worked example from start to finish, and the mistakes that distort the result in practice.
How much to order, and how often
Economic order quantity. The maths runs in your browser.
The formula assumes steady demand and a stable lead time. On a seasonal item or an unreliable supply, it gives an order of magnitude, not an instruction.
Ordering little and often costs a lot in order handling. Ordering rarely and in bulk costs a lot in tied-up stock. The economic order quantity finds the point where the sum of the two is lowest, and that is all it does.
What the formula balances
| If you order | Ordering cost | Holding cost |
|---|---|---|
| Often, in small quantities | High | Low |
| Rarely, in bulk | Low | High |
| At the economic quantity | The sum of both is minimal | |
What the formula cannot do
- It assumes steady demand. On a seasonal item it returns an average matching no real month.
- It ignores volume discounts. A price break at 1,000 units can beat the theoretical optimum.
- It ignores available space and packaging constraints, pallets or supplier minimums.
- It says nothing about stockout risk. That is what safety stock is for.
How to actually use it
- Start with fast-moving items. That is where the gap between current practice and the optimum is worth money.
- Compare the result with what you already do. If the gap is small, change nothing.
- Check it against supplier price breaks before deciding.
- Recalculate when a parameter really moves, not every month.
To run this in a tool rather than a spreadsheet, our 2026 ERP comparison ranks the systems we tested.
Frequently asked questions
What is holding cost?
Everything it costs to hold stock for a year: the capital tied up, storage, insurance, damage and obsolescence. It is expressed as a percentage of the purchase price. Twenty percent is a common benchmark, but it climbs sharply on fragile, bulky or fast-dating goods.
Does it work for seasonal items?
Poorly. It assumes steady demand across the year, which by definition a seasonal item does not have. Reason by season instead, with forecasts and a safety stock sized for the peak.
Should it be applied to every item?
No, that would waste time. Focus on fast-moving items of significant value, the ones ABC analysis classes as A. On the rest, the gap between your current practice and the theoretical optimum will not repay the effort of computing it.