Liquid Nitrogen Dewar Hold Time Calculator
Tell this page how big your tank is and how fast the maker says it loses liquid, and it works out how many days you have left, when to book the next delivery, and whether the room would be safe to walk into if the whole tank let go at once. Nothing uploaded.
When to order more
Oxygen in the room
Level week by week
Straight-line boil-off from today at the working rate. The red rows are below your alarm level.
What a failing vacuum costs you
A dewar keeps its cold with a vacuum jacket, and a jacket that loses its vacuum loses its insulation. This is the usual reason a tank that lasted months suddenly lasts days.
Take it to the tank
Copies the numbers as plain text with the assumptions attached, so a log entry carries its own reasoning.
Learn more: how long liquid nitrogen really lasts in a dewar
Static hold time is one division, and it is the honest starting point
Every vacuum-jacketed dewar has a published static evaporation rate, sometimes printed as normal evaporation rate or NER. It is the litres of liquid the tank loses per day while sitting full, sealed and untouched at room temperature.
Static hold time is that rate divided into the usable capacity. Nothing more sophisticated is going on, and you can check the arithmetic against any datasheet.
Take the Worthington Classic 25, which MiTeGen lists at 25 litres capacity and 0.21 litres a day. Dividing gives 25 ÷ 0.21 = 119.05 days, and the same spec sheet publishes 119 static holding days. The defaults on this page are that tank, so you can see the two figures agree before you put your own numbers in.
Why the spec sheet number is optimistic
The static rate is measured on a tank nobody opens. Every time the lid comes off, warm room air reaches the neck and the cold gas layer above the liquid is lost, and the boil-off rate climbs until the tank settles down again.
No standard defines a working evaporation rate, and manufacturers do not publish one, so this page does not pretend to know yours. The access selector applies a percentage of static hold time that you can see and change, defaulting to 70% for a tank opened about once a day.
Treat that as a placeholder until you have real data. At 0.21 litres a day static and 70% retained, the working rate becomes 0.21 ÷ 0.70 = 0.30 litres a day, and the 119-day tank becomes an 83-day tank.
Measuring your own is easy and beats every assumption here. Note the date and the level at one top-up, note them again at the next, and divide the litres consumed by the days elapsed. Put that figure in the advanced panel as a percentage of static, or just enter it as the evaporation rate directly with the access setting on sealed.
The oxygen check nobody puts on a datasheet
A litre of liquid nitrogen becomes roughly 694 litres of gas near room temperature. The exact multiplier depends on the reference temperature and liquid density a given source picks, which is why the published figures disagree slightly without anyone being wrong.
The University of Iowa's environmental health and safety office states that "liquid nitrogen expands 695 times in volume when it vaporizes", while the University of Texas at Austin puts it at "approximately 696 to 1, (by volume)". The gap between 694 and 696 is under half a percent and changes no decision you would make on it.
Nitrogen is not poisonous. It is dangerous because it displaces the air you need, it is colourless and odourless, and losing consciousness from it comes with very little warning.
OSHA's respiratory protection standard, 29 CFR 1910.134, defines an oxygen deficient atmosphere as "an atmosphere with an oxygen content below 19.5% by volume". Ordinary air is about 20.9%, so the whole margin you have to play with is 1.4 percentage points.
Working the room arithmetic yourself
This page models displacement rather than dilution: the nitrogen pushes air out of a closed room at constant pressure, leaving oxygen at 20.9% × (room volume - gas volume) ÷ room volume. That is the more conservative of the two simple models, which is the right way round for a safety check.
Rearranging it for the room volume that holds the line at 19.5% gives a single number worth remembering: 20.9 ÷ 1.4 = 14.9. A sealed room needs about 14.9 cubic metres of volume for every cubic metre of nitrogen gas released into it.
Run the default tank through that. Twenty-five litres of liquid makes 25 × 694 = 17,350 litres, or 17.35 cubic metres of gas, so staying above 19.5% would take 259 cubic metres of room. A sealed 60 cubic metre room would fall to 14.9% oxygen, and could only absorb 5.8 litres of liquid before crossing the line.
Set against that, routine boil-off barely registers. Three tenths of a litre a day is 208 litres of gas, which moves a 60 cubic metre room from 20.9% to 20.8% oxygen even with the door shut all day.
So the two scenarios in the table say different things on purpose. Day-to-day evaporation in an ordinary room is a non-event, and the case worth planning for is the whole contents leaving at once after a vacuum failure, a spill or a knocked-over tank.
What this calculation deliberately does not do
It assumes no ventilation and perfect mixing. A real room has air changes, and cold nitrogen gas is denser than room air as it boils off, so it pools low before it mixes, which can make the breathing zone near the floor worse than an evenly-mixed average suggests.
That cuts both ways and neither correction belongs in a browser tool. An oxygen monitor in the room measures what is actually there, and mechanical ventilation fixes the underlying problem, so treat the numbers here as a sizing sanity check that tells you whether you need those things.
The hold time side has limits too. Boil-off is treated as a straight line at a constant rate, and real tanks lose a little faster when nearly full and slower when nearly empty. It also cannot see a degrading vacuum jacket, which is the failure mode that actually strands people, so the ladder shows what multiples of the rated rate do to your hold time.
If you are sizing the tank rather than tracking one you own, the solution dilution calculator and the indoor air ventilation calculator cover the neighbouring bench problems.
FAQ
How long does liquid nitrogen last in a dewar?
Divide the usable capacity by the static evaporation rate printed on the spec sheet. A Worthington Classic 25 holds 25 litres and is rated at 0.21 litres a day, which gives 25 divided by 0.21, or 119 days, and 119 static holding days is exactly what the published specification claims. That figure is for a tank nobody touches. Every lid opening lets warm air in, so a tank in real use empties sooner, and the only way to know your own number is to time the gap between two top-ups.
How much nitrogen gas does a litre of liquid nitrogen make?
Roughly 694 litres of gas near room temperature, so a litre of liquid becomes about 0.7 cubic metres of gas. Published safety-office figures run from 694 to 696 depending on the reference temperature and liquid density each one uses, a spread of under half a percent that changes no practical decision. The University of Iowa's environmental health and safety office states that liquid nitrogen expands 695 times in volume when it vaporises, and the University of Texas at Austin puts it at approximately 696 to 1 by volume.
Is my liquid nitrogen tank big enough to be an asphyxiation risk in the room?
Work out the gas volume and compare it against the room. OSHA defines an oxygen deficient atmosphere as one with an oxygen content below 19.5 percent by volume, and holding a sealed room above that line takes about 14.9 cubic metres of room for every cubic metre of nitrogen released. A 25 litre tank makes 17.35 cubic metres of gas, so staying above the line would need 259 cubic metres, far more than most rooms that hold a tank this size. Routine boil-off is not the problem: a fraction of a litre a day is a rounding error in any normal room. The whole contents going at once after a vacuum failure or a knocked-over tank is the case worth planning for, which is why the real answer is an oxygen monitor and moving air rather than arithmetic.