How Long Does Tritium Last? The Real Decay Curve
Tritium does not switch off. It fades, on a fixed schedule, and that schedule is the same in a watch, a rifle sight, a compass or a road marker.
Half-life is 12.32 years. That single number sets everything else, and nothing about storage, use or temperature changes it.
What You Will Actually See, Year by Year
Two figures matter. Decay alone is fixed by the 12.32-year half-life and can be calculated exactly. Real output sits below it, because the phosphor converting the radiation into light also wears out.
| Year | Decay alone | Real output | What you will notice |
|---|---|---|---|
| 0 | 100% | 100% | Full brightness |
| 5 | 75% | 68% | No perceptible change in normal use |
| 10 | 57% | 46% | Visible only beside a new unit in full darkness |
| 12.32 | 50% | 39% | One half-life. Still clearly readable |
| 15 | 43% | 31% | Noticeable to demanding users. Fine for most |
| 20 | 33% | 20% | Dimmer but functional. Replacement worth considering |
| 25 | 24% | 14% | Two half-lives. Visible in genuine darkness |
| 30 | 18% | 10% | Faint. Adequate for detection, not for reading |
The decay curve shown as a graph
The gap between those two curves is the part most guides leave out entirely. It explains why a twenty-year-old device looks dimmer than a decay calculation says it should, and why service life figures from manufacturers sit below the theoretical numbers.
Why the Real Curve Sits Below the Maths
Tritium decay is only half of what dims a device.
The glow comes from beta particles striking a phosphor coating inside the sealed vial. That bombardment never stops, and over years it degrades the phosphor itself. The coating becomes progressively worse at converting the same amount of radiation into visible light.
So two things happen at once. The gas follows its 12.32-year half-life exactly. The phosphor converting it gets steadily less efficient. Observed brightness therefore falls faster than pure decay predicts, and the gap widens with age.
This is why manufacturers quote conservative service lives, and why a twenty-year-old device often looks dimmer than the table suggests. Phosphor formulation and vial construction determine how large that gap becomes, which is one of the real differences between a well-made source and a cheap one. For how tritium illumination works in detail, see our article on tritium lighting explained.
Does Anything Change the Rate?
No, and this is worth stating plainly because the myths are persistent.
Storing it in the dark does nothing. Radioactive decay is indifferent to light. There is nothing to conserve.
Not wearing it does nothing. A watch in a drawer decays at exactly the same rate as one worn daily.
Temperature barely matters. Sealed vials behave consistently across normal operating ranges. Sustained extremes affect the phosphor marginally, not the decay.
Vial size does not slow decay. A larger vial starts brighter and takes roughly one extra half-life to reach the same absolute output. Same rate, higher starting point. If you need twenty years rather than ten, you do not need different tritium. You need to start brighter.
When Yours Reaches End of Life Depends on the Job
There is no universal figure, because the threshold is set by the application rather than the isotope.
Weapon sights have the least tolerance. The sight must be picked up instantly, in darkness, under stress. A meaningful drop matters long before the glow disappears. Ten to fifteen years is a common replacement point. See our tritium sights.
Watches are read deliberately by an eye already dark-adapted, which tolerates far more fade. Fifteen to twenty-five years is normal, and many owners carry on beyond that. See our tritium watches.
Compasses and map readers sit in similar territory. Twelve to twenty years depending on the vial. See our tritium compass and map reader.
Route and hazard markers only need to be detected, not read, so they last longest. Fifteen to twenty-five years. See our tritium road markers.
How to Tell If Yours Is Fading
You cannot judge it alone. The eye adapts to gradual change without registering it, which is why people are often surprised.
The only reliable test is direct comparison. Take the device into a genuinely dark room, let your eyes adapt for several minutes, and hold it beside a new one. The difference is immediately obvious at ten years and stark at twenty.
If you have no new unit to compare against, use the date. Tritium is entirely predictable. If you know when it was made, you know where it sits on the curve.
Replace, or Live With It?
Years 0 to 12. Leave it alone. Output is above half and nothing about the device is limiting you.
Years 12 to 20. Optional. Worth replacing on equipment where instant acquisition matters, particularly sights. Recreational and general use continues comfortably.
Beyond 20 years. Worth a decision for professional use. Around a fifth of original output in practice. Still glowing and still useful for detection, but no longer at the margin a demanding task deserves.
Replacement is usually a matter of fitting new vials rather than discarding the device.
Disposal at End of Life
Tritium devices are sealed sources and should go through regulated channels rather than general waste, even when the glow has almost gone, because the capsule still contains gas. Requirements vary by country, and our team can advise on return and disposal for products we have supplied.
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FREQUENTLY ASKED QUESTIONS
It never fully stops, but practical service life runs 10 to 25 years depending on the application. Decay alone leaves 50% after 12.32 years and 33% at twenty, with real output nearer 20%. Whether that is too dim depends entirely on what you need it for.
No. Tritium produces light continuously through radioactive decay, with no power source, no switch and no light exposure required. It works identically after a week in a drawer as it does in daily use.
Yes, it continues glowing indefinitely at ever-lower output. At thirty years decay alone leaves around 18%, and real output is nearer 10%. That remains detectable in genuine darkness and is adequate for hazard marking, but not for anything requiring rapid reading.
Both decay and phosphor wear are working on them. Sights around twelve years old sit at roughly half their original output from decay alone, with further loss from phosphor degradation. Side by side with a new set the difference is obvious, even though the older sights may still be entirely serviceable on their own.
No. Radioactive decay is unaffected by light, temperature, storage or use. A tritium device in a safe decays at exactly the same rate as one carried every day. Unlike a battery, there is nothing to conserve by not using it.
Refilled, no. The vials are sealed glass capsules and opening one releases the gas. Replaced, usually yes. Fitting new vials restores original brightness while keeping the device, which is normally far cheaper than replacing the whole item.
It reaches the end of its useful life later, but it does not decay more slowly. Half-life is a property of the isotope and is identical regardless of vial size, gas pressure or phosphor colour. A larger vial simply starts brighter, so it takes about one extra half-life to fall to the same absolute output as a smaller one.