Field Service Formulas

How to Calculate Service Parts Demand, First-Time Fix Rate, and MTTR

The five core aftermarket calculations worked end to end, with units, inputs, and where each number comes from.

Service Parts Demand starts from the installed base and a field failure rate. If 10,000 units are in service and a compressor fails at 4 percent per year, expected annual replacements equal 10,000 times 0.04, or 400 units. Multiply by parts per event to catch multi-part repairs: at 1.15 parts per failure you plan for 460 pieces. If only 82 percent of the base is still active or covered, scale by 0.82 to get 377. The Service Parts Demand calculator chains installed base, failure rate, and parts per event so you size annual buys before any safety stock is added on top.

First-Time Fix Rate is first-visit resolutions divided by total service calls. With 1,340 calls closed in a month and 1,140 fixed on the first visit, FTFR equals 1,140 divided by 1,340, or 85.1 percent. The 200 repeat visits each trigger a second truck roll, so they carry into MTTR and Service Call Cost directly. Compute it per product line, not just overall, because a blended 85 percent can hide a 68 percent line that is eating your schedule. The First-Time Fix Rate calculator returns the percentage plus the gap to your target so you can quantify the repeat-visit load.

Mean Time to Repair is total repair time divided by number of repairs. Use clock time on the fix itself, not including travel or parts wait unless your SLA counts them. If 46 repairs consumed 138 hours of wrench time, MTTR is 138 divided by 46, or 3.0 hours per repair. Keep the numerator and denominator in the same window; mixing this quarter's hours with last quarter's repair count skews the result. The Mean Time to Repair (MTTR) calculator handles the division and lets you compare on-site against depot exchange when a part routinely blows the window.

Warranty Reserve per unit equals expected claim probability times average claim cost. If 5 percent of units generate a claim over the term and each claim averages 300 dollars in parts, Field Service Labor Cost, and freight, reserve 0.05 times 300, or 15 dollars per unit sold. Ship 40,000 units and you accrue 600,000 dollars. Add an admin overhead factor, often 8 to 12 percent, if claims processing is not already inside the claim cost. The Warranty Reserve calculator takes units under warranty, cost per unit, claim probability, and overhead, then returns the total accrual you need to book.

Technician Utilization is billable or wrench hours divided by paid hours. A tech paid 173 hours in a month who logs 116 wrench hours runs at 116 divided by 173, or 67 percent. The missing 57 hours are travel, admin, and idle. Decide up front whether you count only invoiced labor or all productive work, because the two definitions can differ by 10 points. The Technician Utilization calculator standardizes the ratio across a crew so you compare people on the same basis rather than on inconsistent timesheet habits.

Parts Fill Rate is demand lines filled from stock divided by total demand lines, and it feeds the fix-rate math above. If a van is asked for 920 parts in a month and stocks 838 of them, fill rate is 838 divided by 920, or 91.1 percent. The 82 unfilled lines are the ones that force a return visit, so fill rate and First-Time Fix Rate move together. The Parts Fill Rate calculator computes this at the van, depot, or central level so you can trace a low fix rate back to the stocking location that caused it.

Service Call Cost is fully loaded cost divided by calls completed, and it is where the other numbers convert to money. If a branch spends 214,000 dollars across labor, travel, and support to close 480 calls, cost per call is roughly 446 dollars. A failed first visit doubles that call, so the FTFR gap you measured earlier maps straight to dollars: 200 repeat visits at 446 each is about 89,000 dollars. The Service Call Cost calculator separates recoverable share and fixed support overhead so you see the true cost behind each dispatch.

Published 2026-07-01.