Safety Stock Calculator
Calculate Safety Stock with your chosen method: fixed days of cover, z × σd × √L, or combined demand and lead-time variability.
In short
Formula: Fixed: SS = daily demand × days of cover. Demand: SS = z × σd × √L. Combined: SS = z × √(L × σd² + d² × σL²).
What this calculator does
Calculate Safety Stock with your chosen method: fixed days of cover, z × σd × √L, or combined demand and lead-time variability. Worked example, questions and limitations included.
Use it to turn Method, Average daily demand, Days of cover (fixed method), Service factor z (your choice — none assumed), and the other shown inputs into a checked result you can compare, copy, or rerun with different assumptions.
The page shows the formula, a numeric worked example, and the assumptions that affect this inventory calculation.
Inputs and what they mean
- Method
- — choice value.
- Average daily demand
- — number value.
- Days of cover (fixed method)
- — number value.
- Service factor z (your choice — none assumed)
- — number value.
- Std. deviation of daily demand
- — number value.
- Average lead time
- — number value.
- Std. deviation of lead time (combined method)
- — number value.
How to use it
- Enter each value in the unit shown next to the box (use the unit converter first if your numbers are in other units).
- Check the breakdown to see every intermediate step.
- Read the limitations before relying on the result.
Formula
Three separate, labelled methods; the calculator never picks one for you.
Statistical methods follow Silver, Pyke and Thomas and the ASCM dictionary; they assume normally distributed, independent demand — a model choice you must confirm.
z is entered by you (e.g. 1.65 is commonly used for about 95% cycle service under normality). No service level is assumed.
Fixed: SS = daily demand × days of cover. Demand: SS = z × σd × √L. Combined: SS = z × √(L × σd² + d² × σL²).
Inputs used: Method, Average daily demand, Days of cover (fixed method), Service factor z (your choice — none assumed), Std. deviation of daily demand, Average lead time, Std. deviation of lead time (combined method).
Worked example
Demand variability method
- √9 = 3.
- 1.65 × 10 × 3 = 49.5 units.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Limitations and assumptions
- Normal distribution is a model assumption.
- Daily periods assumed independent.
- No review-period term (continuous review).
- Results are planning estimates built only from the figures you enter. No prices, rates, demand patterns or service levels are built in, and the results are not supplier quotes or carrier rates.
- The result depends on the values you enter for this safety stock calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.
Common questions
How do I choose z?
It follows from the cycle service level you decide on. The service level calculator shows the probability a given z represents under a normal model.
My demand is lumpy or intermittent. Is this valid?
Probably not. Normal-based formulas fit poorly for slow or lumpy items; use a method validated for your data.
Which method should I use?
Combined if supplier lead times vary, demand-only if they are reliable, fixed cover if you lack history. Check the formula, example, and limitations on this page before using the result for a real inventory decision.
How do I use the Safety Stock Calculator?
Enter the required values for Method, Average daily demand, Days of cover (fixed method), Service factor z (your choice — none assumed), Std. deviation of daily demand, and the other fields shown. The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.
What formula does the Safety Stock Calculator use?
Fixed: SS = daily demand × days of cover. Demand: SS = z × σd × √L. Combined: SS = z × √(L × σd² + d² × σL²). The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.
Can the Safety Stock Calculator be used for exact decisions?
Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.
Related tools
Calculate EOQ step by step: EOQ = √(2 × annual demand × cost per order ÷ holding cost per unit per year).
Calculate Reorder Point step by step: reorder point = average daily demand × lead time + safety stock.
Calculate Inventory Turnover step by step: cost of goods sold ÷ average inventory.
Calculate Inventory Days step by step: average inventory ÷ cost of goods sold × days in period.
Calculate Carrying Cost step by step: capital cost + storage + insurance and taxes + obsolescence and shrinkage.
Calculate Stock Coverage step by step: (on hand + on order) ÷ expected daily demand.