Lead Time Calculator: calculate lead time for your business. Formula, benchmarks, and practical tips included.
A supply chain calculator covers the three pillars of inventory management: Economic Order Quantity (EOQ) to minimise total ordering and holding costs, safety stock and reorder point to prevent stockouts during lead time, and seasonal demand forecasting using seasonal indices. These techniques are standard tools in operations management, procurement, and supply chain planning for businesses of all sizes.
The EOQ model (Wilson formula) finds the optimal order quantity that balances the cost of ordering too often (high ordering cost) against the cost of holding too much inventory (high holding cost). Safety stock adds a buffer above the average lead-time demand to absorb variability in demand or lead time. Seasonal indices decompose annual demand into quarterly or monthly patterns to guide procurement planning.
For a related business workflow, compare Safety Stock Calculator and Fill Rate Calculator. They sit in the same planning family but answer different questions, so keep the period, denominator, units, and decision boundary visible when moving between calculators.
Inventory decisions work best when the demand history, valuation basis, lead-time definition, review cadence, and service promise are recorded together. Annual demand is not the same as a recent shipment spike; a supplier lead time is not the same as the internal approval delay; and a service target is not a guarantee that every order will arrive on time. Keep stockout consequences, minimum order quantities, storage limits, obsolescence risk, supplier constraints, and criticality exceptions beside the numerical result. Run a low-demand and high-demand scenario when the inputs are uncertain, and preserve the original list or time series so a later review can distinguish a real operational change from a changed measurement boundary.
Use the calculator output as one row in an operating review. Compare it with supplier performance, open orders, forecast error, carrying cost, storage capacity, and the service failures that matter most to customers. Recalculate after a product, supplier, price, or fulfilment-policy change, and explain any override rather than hiding it in a rounded input. Keep approval ownership clear for urgent buys, substitutions, and deliberate service trade-offs.
Economic Order Quantity (Wilson/Harris): EOQ = √(2DS/H), where D = annual demand, S = cost per order, H = annual holding cost per unit. Total annual cost = (D/EOQ)S + (EOQ/2)H.
Safety stock: SS = Z × σ_d × √(LT), where Z is the service-level z-score (90% → 1.282; 95% → 1.645; 99% → 2.326), σ_d = standard deviation of daily demand, LT = lead time in days. Reorder point: ROP = d̄ × LT + SS.
Seasonal index = quarter demand / quarterly average. Next year's quarterly forecast = annual forecast × seasonal index / 4.
EOQ minimises total inventory cost — but only under its simplifying assumptions (constant demand, fixed lead time, no volume discounts). In practice, MOQ constraints, supplier lead-time variability, and perishability all require adjustments. A 95% service level means you will have stock available in 95 out of 100 order cycles; the remaining 5% risk a stockout. Increasing to 99% roughly doubles safety stock due to the non-linear increase in z-score. For low-value, high-demand items, a lower service level (90%) may be cost-effective; for critical or expensive items, 99%+ is standard.
EOQ and safety stock formulas are standard operations research models. The z-scores used are based on the standard normal distribution. Seasonal index calculations assume additive seasonality and equal-length periods. This calculator is for planning and estimation purposes only; actual procurement decisions should account for supplier constraints, contractual minimum order quantities, and business-specific risk tolerances.