Maintenance Math

How to Calculate CMMS, EAM, and Spare Parts Metrics: Formulas and Worked Examples

The core maintenance and spare parts formulas worked line by line, with real inputs, units, and where each number comes from.

Start with wrench time, the fraction of a technician's paid hours spent turning a wrench versus waiting, walking, or searching. The formula is wrench time equals direct hands-on hours divided by total paid hours. If a tech logs 8.0 paid hours and only 2.6 go to actual repair, wrench time is 2.6 divided by 8.0, or 32.5 percent. Pull the hands-on number from work order labor stamps, not self-reports, which inflate by 15 to 25 percent. The Wrench Time calculator lets you subtract travel, parts staging, and permitting so you isolate true tool time from the padding that hides in most timesheets.

Planned maintenance compliance measures whether scheduled PMs actually got done in their window. The formula is PMs completed on schedule divided by PMs scheduled, over a fixed period. If 420 PMs were due in June and 357 closed before their due date, compliance is 357 divided by 420, or 85.0 percent. Count a PM as compliant only if closed within the grace window, typically 10 percent of the interval, so a 30 day PM has a 3 day tail. The Planned Maintenance Compliance calculator handles the grace math and separates true completions from work orders force-closed without labor.

Work order backlog is measured in crew-weeks, not raw counts, because a count ignores job size. The formula is total estimated labor hours of ready backlog divided by weekly available craft hours. Suppose 1,240 estimated hours sit in ready backlog and your crew supplies 310 productive craft hours per week. Backlog is 1,240 divided by 310, or 4.0 crew-weeks. Only count work that is planned, parts-staged, and ready to schedule; jobs waiting on engineering are not backlog. The Maintenance Work Order Backlog calculator splits ready from total so you size the true queue.

Spare parts carrying cost is the annual penalty for holding inventory. The formula is average inventory value multiplied by the carrying rate, where the rate sums capital cost, storage, insurance, obsolescence, and shrink. A storeroom holding 1.8 million dollars at a 24 percent carrying rate costs 1,800,000 times 0.24, or 432,000 dollars per year. The rate rarely drops below 18 percent and often hits 30 percent for slow-moving MRO. Break the rate into its parts so you know which lever moves it. The Spare Parts Carrying Cost calculator itemizes each component so you can attack obsolescence separately from cost of capital.

Critical spares coverage answers whether you hold enough of the parts that stop production. The formula is critical SKUs with on-hand quantity at or above minimum divided by total critical SKUs. If 214 of 250 critical SKUs are stocked to their minimum, coverage is 214 divided by 250, or 85.6 percent. Define the minimum from lead time times average usage plus a safety buffer sized to demand variability. For a pump seal with a 6 week lead time and usage of 0.5 per week, the reorder point is 6 times 0.5 plus safety, roughly 4 units. The Critical Spares Coverage calculator flags every SKU below its point.

Stockout downtime exposure converts a missing part into an expected dollar loss. The formula is stockout probability multiplied by mean downtime hours multiplied by the hourly cost of the affected line. A part with a 12 percent annual stockout chance, an 18 hour repair-plus-shipping delay, and a line worth 4,200 dollars per hour carries exposure of 0.12 times 18 times 4,200, or 9,072 dollars per year. Compare that against the carrying cost of stocking one more unit to make a rational stock-or-not call. The Stockout Downtime Exposure calculator runs this trade for every critical SKU in seconds.

Maintenance labor load checks whether committed work fits available capacity. The formula is total scheduled labor hours divided by net available craft hours, expressed as a percent. If you schedule 372 hours against 310 available, load is 372 divided by 310, or 120 percent, meaning you are 62 hours overcommitted and will slip PMs. Net available means gross hours minus vacation, training, and the unplanned break-in reserve, usually 20 to 30 percent. The Maintenance Labor Load calculator nets these deductions so you schedule to real capacity, not headcount times 40.

Asset hierarchy completeness gauges whether your data can even support these metrics. The formula is fully attributed asset records divided by total asset records, where fully attributed means parent, criticality, and BOM link present. If 1,540 of 2,000 assets carry all three fields, completeness is 77.0 percent, and the missing 460 will corrupt every downstream KPI. Aim to close the gap before trusting backlog or coverage numbers, since a part with no BOM link cannot be flagged critical. The Asset Hierarchy Completeness calculator audits each record so you know which fields to fix first.

Published 2026-07-01.