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
- Min stock level = daily demand × lead-time horizon
- Max stock level = min level × service-level buffer ÷ record accuracy
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 lead-time horizon for the min level, scale by the service buffer, then divide by record accuracy - phantom stock means planning MORE, not less. With 6.5 parts/day over 45 days the min is 292.5 units, and the max is 292.5 x 1.25 / 0.95 = 384.87 units.
- What is the difference between min and max in this policy? The max is the order-up-to level this calculator sizes (here 384.87 units). The min, or reorder point, is demand during the lead-time horizon - 292.5 units here - the level at which you trigger replenishment. 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 2026-08-11.