Estimate ratio calculator
Most people estimate from the best case. Put in what you guessed and what it really took for your last few jobs, and get your own ratio and a range to quote next time.
Your ratio appears here
Fill in the estimate and the real time for at least 3 jobs.
Your estimate ratio
For the next job
Keep the record going: write the estimate in the task's note when you plan a job and the real time when you tick it, in tick-tock-done.
How it works
For each job the ratio is the real time divided by your estimate: 1.5 means it took half as long again as you said. Your ratio is the middle one (the median), so one disaster job doesn't drag it around. The likely time for the next job is your estimate times that ratio; the safer number uses the ratio three quarters of your jobs came in under.
With three to ten jobs this is a rough guide, not a forecast. It works best when the jobs are of one kind: price a bathroom from bathrooms, not from fences.
Why estimates run short, and what helps besides a ratio, is in The planning fallacy: why every task takes longer than you think.