Relative Time Calculator: How Long Does an Hour Feel at Another Age?

Free relative time calculator: see how long a span of time feels at another age. One hour for a 9-week-old baby is the same share of life as 16.9 days for a 70-year-old. Compare the share-of-life and square-root models, see how much of your felt life is behind you, and compare animal lifespans.

How long

for someone aged

feels like how long at age

How time feels with age

Custom: 1.00

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The same slice of two lives

How much of your felt life is behind you?

Lifespan (years)

Memories start at age

Move over the chart to read any age. The bold line is the model you picked; the thin lines are the other two. Felt time is counted from the age your lasting memories start, because almost nobody remembers being 2.

This span at other ages

The maths

share of life (Janet) \( \displaystyle d_B = d_A \times \frac{B}{A} \)

any model: same felt length \( \displaystyle \int_A^{A+d_A} t^{-k}\,dt = \int_B^{B+d_B} t^{-k}\,dt \)

short spans \( \displaystyle d_B \approx d_A \left(\frac{B}{A}\right)^{k} \)

felt life used by age a (k = 1) \( \displaystyle \frac{\ln(a/a_0)}{\ln(L/a_0)} \)

k = 1 is the share-of-life rule, k = 0.5 the square-root model and k = 0 plain clock time. For k = 1 the integral gives exactly d_A/A = d_B/B, so “share of the life lived so far” and “time felt in proportion to age” are the same rule, even for long spans.

How the relative time calculator works

A popular idea, first written down by the French philosopher Paul Janet in 1877, is that a stretch of time feels as long as its share of the life you have lived so far. A 9-week-old baby has lived 63 days, so one hour is 1/1,512 of its whole life. For a 70-year-old, 1/1,512 of their life is about 16.9 days. In that sense an hour for the baby is “as long” as two and a half weeks for the grandparent.

The calculator also has a gentler square-root model (Lemlich, 1975), where felt time shrinks with the square root of age (the same hour is then about 20 hours for the 70-year-old), plain clock time for comparison, and a slider for anything in between.

Is this real?

It is a model, not a measurement. People of all ages say the years seem to go faster as they get older, and studies agree that it feels that way looking back, but they do not find that an older person's hour or day actually drags or flies by. Likely reasons include fewer new experiences (new events make time feel longer in memory), routine, and attention. Use the numbers as a way to think about time, not as a scientific fact.

Questions

What can I type?: Things like 1 hour, 90 min, 1h 30m, 1:30, 9 weeks, 2.5 years, a summer or half an hour. A bare number means hours for the length and years for an age.
Why does a summer feel endless to a child?: For a 6-year-old, 3 months is 1/24 of their life. The same share of a 40-year-old's life is 20 months.
What is the “felt life” chart?: It adds up felt time from the age your memories start. With the share-of-life rule, memories from age 5 and an 80-year lifespan, half of your felt life is used by age 20, the geometric mean of 5 and 80.
What about animals?: The “Two lifespans” tab compares a span as a share of a whole typical lifespan: one day of a 12-year dog life is about 6.6 days of a 79-year human life. Lifespans are rough averages and vary a lot by breed and species.
Is anything sent anywhere?: No. Everything runs in your browser; your last numbers are kept in this browser only.

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