CMMS, EAM & Spare Parts Management calculator
Spare Parts Min Max Calculator
Min-max is the workhorse reorder policy in nearly every CMMS storeroom: when on-hand stock drops to the minimum, you replenish back up to the maximum. This calculator sizes the maximum level from the parts you actually consume, how long replenishment really takes, and how confident you want to be that the part is on the shelf when a technician needs it. Reliability engineers, storeroom supervisors, and MRO buyers use it to stop two expensive failure modes at once: stockouts that turn a planned job into a breakdown, and dead capital sitting on shelves. Because it folds in record accuracy, it also accounts for the gap between what the system says you have and what is physically there.
What this calculator does
- Estimate an adjusted maximum stocking level from average parts demand, replenishment horizon, service-level buffer, and inventory accuracy.
- a maintenance or asset-management team needs to set min-max levels that protect uptime without tying up unnecessary MRO inventory for a spare parts min-max review
- It calculates a gross min-max stock level from daily demand and lead-time cycles, then a usable level after applying your service-level buffer and storeroom record accuracy.
Formula used
- Gross spare parts min max = average spare parts demand per day × maximum replenishment horizon
- Usable spare parts min max = gross spare parts min max × desired service-level buffer × storeroom inventory accuracy factor
Inputs explained
- Average spare parts demand per day:
- Maximum replenishment lead-time horizon:
- Target service-level buffer:
- Storeroom inventory record accuracy:
How to use the result
- Use it when setting or auditing max stock levels for MRO and spare parts, especially after a demand shift, a supplier lead-time change, or a cycle-count program that exposes record accuracy.
- It assumes demand is roughly steady; for lumpy, single-failure-driven spares (large bearings, motors), a criticality and Poisson-based model will size stock better than averaged demand.
Common questions
- How do you calculate a spare parts min-max level? Multiply average daily demand by the replenishment horizon to get gross stock, then scale by your service-level buffer and inventory accuracy. With 6.5 parts/day over 45 cycles you get a gross 292.5 units, which becomes 347.34 usable units after the 125% service factor and 95% accuracy adjustment.
- What is the difference between min and max in this policy? The max is the order-up-to level this calculator sizes (here 347.34 units of working capacity). The min, or reorder point, is the level at which you trigger replenishment and is typically demand-during-lead-time plus safety stock. You replenish from min back up to max.
- What is a good service-level buffer to use? For non-critical, easily sourced parts, 90-110% is reasonable; for critical spares where a stockout causes downtime, 120-150% is common. The default 125% reflects a part you do not want to be caught short on.
- Why does inventory accuracy lower my usable stock? If your records are 95% accurate, roughly 5% of the on-hand figure is phantom stock that is not really there. The calculator discounts for that so your effective shelf availability matches the target rather than the optimistic system number.
- How often should I recompute min-max levels? Review quarterly for high-movers and after any event that changes demand or lead time. Min-max set once and never revisited is the single most common cause of both stockouts and obsolete inventory in MRO storerooms.
Last reviewed 2026-05-12.