TAS 2% Rule-of-Thumb Trainer
The mental-math TAS rule — add 2% per 1,000 ft — run side-by-side against the exact density calculation so you learn exactly when to trust it.
The rule rides the pressure altitude; reality rides the density altitude. On ISA days they agree within a knot or two — on a 25 °C-above-ISA afternoon, the rule under-reads TAS by several knots.
Formula
⚠️ For flight planning and education only — verify with your POH/AFM, certified instruments and official sources. Not for primary navigation or airworthiness decisions.
The mental-math TAS rule — add 2% per 1,000 ft — run side-by-side against the exact density calculation so you learn exactly when to trust it.
About TAS 2% Rule-of-Thumb Trainer
Mental math keeps you ahead of the airplane, but only if you know each shortcut's failure mode. This trainer runs the beloved 2%-per-1,000-ft TAS rule against the exact density-altitude computation simultaneously, displaying the signed error — teaching, through repetition with your own numbers, that the rule is excellent near standard temperature and quietly optimistic on hot days.
How to use TAS 2% Rule-of-Thumb Trainer
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula rule: TAS ≈ CAS × (1 + 0.02 × PA/1000); exact: TAS = CAS/√σ(DA) substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use TAS 2% Rule-of-Thumb Trainer?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula rule: TAS ≈ CAS × (1 + 0.02 × PA/1000); exact: TAS = CAS/√σ(DA) with sources cited on the page
- ✓The rule rides the pressure altitude; reality rides the density altitude. On ISA days they agree within a knot or two — on a 25 °C-above-ISA afternoon, the rule under-reads TAS by several knots.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
How do I run the 2% rule in my head quickly?+
Double the altitude in thousands to get the percentage, then apply it. At 9,000 ft: 18%. For 120 knots, 10% is 12 and 8% is about 10 — call it +22, so 142 kt. With practice the whole chain takes five seconds, which is the entire point of the rule.
When does the rule break down?+
Two regimes: strongly non-ISA temperatures (it has no temperature input, so a hot day's extra density altitude is invisible to it), and above roughly 15,000 ft where the linearization sags below the true exponential. Within the GA cruise band on a near-standard day it stays inside 2 knots.
Is there a better mental rule for hot days?+
Add 1% per 1,000 ft of density altitude divided by... simpler: estimate density altitude first (PA + 120 ft per °C above ISA), then apply 2% per 1,000 ft of that. It's one extra step and recovers most of the temperature error — this tool lets you verify the upgrade with real numbers.
Why learn the rule at all when calculators exist?+
Cross-checking. Glass cockpits display TAS computed from air data, and the day a clogging pitot or failed temp probe skews it, the pilot with a calibrated mental estimate notices in seconds. Rules of thumb are instrument-failure insurance that weighs nothing.
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