What is the difference between MTBF and MTTF?

MTBF applies to repairable items and measures time between failures. MTTF applies to non-repairable items — a bearing, a lamp, a fuse — and measures time to the single failure that ends its life.

Does higher MTBF always mean higher availability?

No. Availability depends on both MTBF and restoration time. An asset with a long MTBF but a two-week repair can have lower availability than one that fails more often but is back in twenty minutes.

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MTBF & Availability Calculator

Availability answers a blunt question: what fraction of the time is the asset able to do its job? It falls out of two measurements — how long the asset runs between failures (MTBF) and how long it takes to restore (MTTR). Improving either raises availability, and knowing which one dominates tells you whether to invest in reliability or in maintainability.

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Formula

Inherent availabilityAi = MTBF / (MTBF + MTTR)

Operational availabilityAo = MTBF / (MTBF + MTTR + MLDT)

MLDT is mean logistics delay time — waiting for parts, crew or a process window.

Failure rate from MTBFλ = 1 / MTBF

Exponential reliabilityR(t) = exp(−t / MTBF)

Valid only when the failure rate is constant.

Annual downtimeDowntime = (1 − A) × 8760 hours

Worked example

A compressor records MTBF = 850 operating hours and MTTR = 6 hours.

Ai = 850 / (850 + 6) = 850 / 856 = 0.99299
Availability = 99.30%
Annual downtime = (1 − 0.99299) × 8760 = 61.4 hours
λ = 1 / 850 = 1.176 × 10⁻³ failures per hour

The unit loses roughly 61 hours a year. Halving MTTR to 3 hours recovers about 31 of those hours — usually far cheaper than doubling MTBF to achieve the same result.

Run this with your own numbers

Common mistakes

Applying R(t) = exp(−t/MTBF) to an item that wears out. The exponential model assumes a constant failure rate; for wear-out items use Weibull instead.
Mixing calendar hours and operating hours between MTBF and MTTR. Pick one basis and hold it.
Quoting inherent availability to an operations audience. They experience operational availability, which includes parts and crew delays and is always lower.
Averaging MTBF across dissimilar assets. The result describes nothing that actually exists.

Frequently asked questions

What is the difference between MTBF and MTTF?: MTBF applies to repairable items and measures time between failures. MTTF applies to non-repairable items — a bearing, a lamp, a fuse — and measures time to the single failure that ends its life.
Does higher MTBF always mean higher availability?: No. Availability depends on both MTBF and restoration time. An asset with a long MTBF but a two-week repair can have lower availability than one that fails more often but is back in twenty minutes.

About Reliability Pro

Availability & MTBF is part of a suite of 126 reliability, maintenance and quality engineering tools covering life data analysis, accelerated testing, system reliability, FMEA and root cause, SPC, and design for reliability. It runs in the browser and as native iOS and Android apps, so the same calculation is available at a desk or in front of the asset.

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