Teams & projects
Estimate accuracy calculator
Add up your estimated and actual hours from recent tasks; get your personal correction factor and apply it to the next estimate you give.
Use at least 5–10 finished tasks of a similar kind; one task is too noisy to calibrate on.
How the estimate multiplier is calculated
Your personal multiplier is the ratio of reality to prediction: multiplier = actual hours ÷ estimated hours, summed over a batch of finished tasks. The error percentage is (actual − estimated) ÷ estimated × 100. To correct a fresh guess, multiply it by the factor. Summing first and dividing once matters: averaging per-task ratios lets a 15-minute job that took 45 minutes (a 3× ratio) distort the result as much as a two-day project.
Worked example: last month you estimated your tasks at 40 hours in total and they took 52. The multiplier is 52 ÷ 40 = 1.3, meaning you run 30% over. A new task you instinctively call 10 hours should be planned as 10 × 1.3 = 13 hours, a 3-hour buffer that comes from your own data rather than a guess.
| Sample (est. → actual) | Multiplier | Error | An 8 h guess becomes |
|---|---|---|---|
| 20 h → 18 h | 0.90 | −10% | 7.2 h |
| 20 h → 22 h | 1.10 | +10% | 8.8 h |
| 20 h → 25 h | 1.25 | +25% | 10 h |
| 20 h → 30 h | 1.50 | +50% | 12 h |
| 20 h → 40 h | 2.00 | +100% | 16 h |
Reading your result and improving it
Underestimating is the normal direction. Psychologists call it the planning fallacy: in a study by Buehler, Griffin and Ross (1994), students predicted they would finish their theses in 33.9 days on average, and the real average was 55.5 days. A multiplier between 0.9 and 1.1 is excellent calibration; 1.2 to 1.5 is common; above 1.5 usually signals that tasks are too big to estimate as one block or that interruptions are not being counted.
Keep separate multipliers for different kinds of work. Writing, coding, admin and anything that depends on other people tend to have very different factors, and mixing them hides the pattern. Record the estimate before you start, not afterwards, or memory quietly rewrites it. Recalculate every few weeks: as you apply the factor, your raw guesses often shift, so the multiplier should drift towards 1. A factor below 0.9 is a cue to stop padding, since inflated plans waste slots in your calendar just as overruns do.
Frequently asked questions
How many tasks do I need to calculate my estimate multiplier?
Aim for at least 5 to 10 completed tasks of similar type. With fewer, one unusual task can swing the ratio a lot; with 20 or more the factor becomes quite stable.
Should I multiply or add a fixed buffer to my estimates?
Multiplying is usually better because overruns grow with task size. A fixed buffer of one hour is too much for a 30-minute job and far too little for a 3-day project.
Why do I keep underestimating how long tasks take?
People picture the ideal path and forget interruptions, rework, waiting and setup. Breaking a task into steps and estimating each one, then applying your multiplier, reduces the gap.