Free tool
Tach to Hobbs converter
Tach time converts to Hobbs time by dividing it by the aircraft's Tach/Hobbs ratio — the tach total divided by the Hobbs total over the same flights. AOPA puts tach time typically 10 to 20 percent below Hobbs time, so the ratio usually sits between 0.80 and 0.90. Convert hours and hourly rates both ways below, or work out your own aircraft's ratio.
1. Your aircraft's Tach/Hobbs ratio
Tach hours divided by Hobbs hours over the same flights. AOPA puts Tach time typically 10 to 20 percent below Hobbs time — a ratio of 0.80 to 0.90. The converter starts at 0.85; work out your own in step 4 or 5.
Tach reads 15% below Hobbs
2. Convert Tach time to Hobbs time (and back)
Type in either field. Hobbs hours × ratio = Tach hours; Tach hours ÷ ratio = Hobbs hours.
3. Convert an hourly rate
The rate that collects the same money on the other meter. Tach rate = Hobbs rate ÷ ratio. Any currency.
4. Find your ratio from your own logs
Add up both meters over the same flights — a season of flight logs, or the difference between two readings of each meter — and enter the totals.
Your ratio appears here.
5. Estimate one flight from its RPM
No logs yet? Describe a typical flight. The model here assumes a Hobbs wired to oil pressure, which counts every minute the engine runs, and a tach hour meter that counts in proportion to RPM and reads one hour per hour at its reference RPM. That reference differs per instrument — 2,300 is an example, not your aircraft's value; check the tachometer or ask your maintenance shop.
| Phase | Minutes | RPM |
|---|---|---|
| Start-up and taxi out | ||
| Run-up | ||
| Take-off and climb | ||
| Cruise | ||
| Descent and circuit | ||
| Taxi in and shut-down |
Hobbs 1.17 h · Tach 0.92 h · ratio 0.79
Hobbs hours in tach hours at a ratio of 0.80, 0.85 and 0.90
The same Hobbs time reads as fewer tach hours the more of it is spent at low RPM. The three columns span AOPA's typical range; read the column closest to your aircraft's own ratio.
| Hobbs h | Tach h at 0.80 | Tach h at 0.85 | Tach h at 0.90 |
|---|---|---|---|
| 0.5 | 0.40 | 0.43 | 0.45 |
| 1.0 | 0.80 | 0.85 | 0.90 |
| 1.5 | 1.20 | 1.28 | 1.35 |
| 2.0 | 1.60 | 1.70 | 1.80 |
| 10.0 | 8.00 | 8.50 | 9.00 |
| 50.0 | 40.00 | 42.50 | 45.00 |
| 100.0 | 80.00 | 85.00 | 90.00 |
Meters show tenths of an hour, so on one short flight the rounding alone can move the ratio by several hundredths. Work out a ratio over many flights, not one.
Bill on the meter your club chose
In Pilot-Next each aircraft has a default billing meter, Hobbs or Tacho, and an administrator can set a different one per pilot. The pilot enters the meter readings when settling the flight, and the hours times the rate become one line on the pilot's account — no spreadsheet, and both readings stay on the flight log when the aircraft records both meters.
Read more: Hobbs vs Tacho — which meter nine of 17 flying clubs bill on, wet vs dry rates, how a flying club should bill its pilots for flight time and what one flight hour costs the owners.
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Tach and Hobbs time: questions
Divide the tach hours by the aircraft's Tach/Hobbs ratio: Hobbs hours = Tach hours ÷ ratio, and Tach hours = Hobbs hours × ratio. The ratio is the aircraft's tach total divided by its Hobbs total over the same flights. AOPA puts tach time typically 10 to 20 percent below Hobbs time, a ratio of 0.80 to 0.90; at 0.85, 85 tach hours equal 100 Hobbs hours. There is no fixed conversion factor, because the ratio depends on how the aircraft is flown.
A Hobbs meter counts clock time — depending on the installation, while an oil-pressure switch senses the engine running, while the master switch is on, or only when an airspeed or landing-gear squat switch says the aircraft is flying. A tachometer hour meter counts engine revolutions, so it runs slower at low RPM: taxi, run-up, pattern work and slow flight add less tach time than Hobbs time. On a training flight with a lot of slow flight or pattern work the tach time is lower.
Divide the Hobbs rate by the aircraft's Tach/Hobbs ratio. At a ratio of 0.85, a rate of 180 per Hobbs hour becomes 211.76 per tach hour, and both collect the same money on the same flights. A club that switches meters and keeps its old rate collects 15 percent less at that ratio; AOPA notes that a rental rate based on the tach would be at least 10 percent higher.
Not by definition. For maintenance records, 14 CFR 1.1 defines time in service as the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing. A Hobbs meter on an oil-pressure switch also counts taxi time, and a tach counts revolutions rather than time, so neither measures it exactly. AOPA notes that most flight schools bill rental on the Hobbs meter and use the tachometer for engine service intervals. Which reading an aircraft uses for its maintenance records is a question for its maintenance organisation.
Yes. Each aircraft in Pilot-Next has a default billing meter, Hobbs or Tacho, and an administrator can set a different billing meter per pilot. An aircraft that records both meters keeps both readings on every flight log, which is also how a club finds its own Tach/Hobbs ratio over a season.
Sources
- Mike Collins, “Understanding the Hobbs meter”, AOPA Flight Training, 1 September 2018 — how a Hobbs meter is switched, tach time typically 10 to 20 percent lower, schools billing on Hobbs and servicing on the tach.
- 14 CFR 1.1, definitions of time in service and flight time.
This converter gives an estimate from the ratio you enter. It is not maintenance or airworthiness advice: the aircraft's own records and your maintenance organisation have the final word.