An inhaler does not get visibly emptier. You cannot see through the canister, it weighs next to nothing either way, and it keeps making a satisfying hiss for a while after the medicine is gone. So the question of how long it will last has to be answered with a number, not a feeling.
The good news is that the number is easy to get.
The basic sum
Days left = puffs left ÷ puffs per day, rounded down to whole days.
Puffs per day is puffs per dose times doses per day. Two puffs twice a day is four puffs a day. One puff twice a day is two.
The worked example most people want: an inhaler with 200 puffs, used at 2 puffs twice a day, uses 4 puffs a day and lasts 50 days.
Round down, not to the nearest. If the division leaves a remainder, those last few puffs are not a full day's use. They are the part of the inhaler you should not be relying on.
The inhaler days calculator does this division and turns it into a date — the last full day and the day to reorder by — which is the form you can actually put in a calendar.
How many puffs are in an inhaler?
It depends on the inhaler, and the only reliable answer is the one printed on yours. Look on the box, the canister label or the leaflet for the number of doses or actuations. Common labelled counts include 60, 120 and 200, but there are others, and two inhalers that look alike can hold very different amounts.
Here is how long those common sizes last at a few rates of use:
| Puffs on the label | 1 puff a day | 2 a day | 4 a day | 6 a day | 8 a day |
|---|---|---|---|---|---|
| 60 | 60 days | 30 days | 15 days | 10 days | 7 days |
| 120 | 120 days | 60 days | 30 days | 20 days | 15 days |
| 200 | 200 days | 100 days | 50 days | 33 days | 25 days |
Two of those cells do not divide evenly. 60 puffs at 8 a day is 7.5, so 7 full days. 200 puffs at 6 a day is 33.3, so 33 full days — 33 × 6 is 198, which leaves 2 puffs that do not make a day.
Use the dose counter
Many inhalers now have a built-in dose counter, and if yours does, it is the most reliable thing you have. It counts each actuation, whether that puff went into your lungs, into the air while priming, or into your sleeve when the cap was knocked.
Start from the counter's reading, not from the number on the box. An inhaler that has been in use for a fortnight is not a 200-puff inhaler any more.
Counters differ. Some count down one at a time, some only change every ten or twenty puffs, and some change colour as they approach the end. The leaflet will say how yours works, and it is worth two minutes to read it once so you know what "nearly empty" looks like before you get there.
If yours has no counter, the leaflet will usually suggest a way of keeping track — often marking the doses down as you use them. Pharmacists will also happily explain the options for your specific inhaler.
Why you should not guess
Two old methods still circulate: floating the canister in water to see how full it is, and shaking it to hear whether there is anything left.
Neither is a reliable way to tell how much is left. (Shaking before each puff, if your leaflet says to, is a different thing: that mixes the medicine, and you should keep doing it.) Shaking to listen tells you something is in there — but an inhaler can keep spraying propellant after it has stopped delivering a full dose of medicine. Putting the canister in water is something many leaflets specifically tell you not to do, and it does not tell you about the medicine either.
Many inhaler leaflets say to throw the inhaler away once the labelled number of doses has been used, even if it still seems to spray. That is the reason: the puffs after the labelled count are not guaranteed to contain what they should. Count, or use the counter.
Priming uses puffs too
Many inhalers need priming — spraying a few puffs into the air — before first use, and sometimes again if they have not been used for a while or have been dropped. How many puffs, and when, varies by product, so check your leaflet.
Those puffs come out of the total. If a label said to prime with, say, four sprays, a 200-puff inhaler would have 196 left for use. At 4 puffs a day, that is 49 days rather than 50.
A day's difference sounds small. If you re-prime regularly, it adds up, and the counter is the only thing that captures it without you having to remember.
A dated example
Your counter reads 200 on 1 October. You use 2 puffs twice a day, so 4 a day, and you want 7 days of buffer before you run out.
- Days left: 200 ÷ 4 = 50 days.
- Last full day: 1 October plus 49 days = 19 November. (1 October is day one, so the fiftieth day is 49 days later: the 31 days of October, then 19 in November.)
- Reorder by: 19 November minus 7 = 12 November.
A week of buffer covers a weekend, a pharmacy that has to order the inhaler in, and a prescription that needs a quick renewal. If yours needs a review appointment first, leave more.
Reliever inhalers are harder to predict
A preventer used on a fixed schedule is easy: the rate is the same every day, so the sum is exact. A reliever used as needed is different, because the rate depends on how you have been.
The honest way to estimate it is from your recent use, not a guess. Look at the counter twice, a couple of weeks apart:
- On the first reading it says 200. Fourteen days later it says 172.
- That is 28 puffs in 14 days, an average of 2 a day.
- At that rate, 172 puffs would last 86 days.
Treat that as a rough guide, not a date to rely on. A bad week can use several weeks' worth, and a reliever is exactly the inhaler you do not want to find empty when you need it. Many people keep a spare for that reason — worth asking your pharmacist or prescriber about.
If you notice you are using a reliever noticeably more often than usual, that is worth mentioning to your doctor or nurse. How much is too much is a question for them, not for a calculator.
Other things that end an inhaler early
The puff count is not the only limit.
- Expiry dates. Every inhaler has one, and some have a shorter use-by once opened. The leaflet will say.
- Damage or blockage. A blocked or damaged inhaler can stop working properly well before it is empty; the leaflet explains cleaning and when to replace it.
- Losing track of which one is which. If you have a preventer and a reliever, the numbers for one say nothing about the other. Work each out separately.
The short version
Puffs left divided by puffs per day, rounded down, is how many days an inhaler lasts. 200 puffs at 2 puffs twice a day is 50 days. Start from the counter, which already counts any priming sprays; if you are working from the box instead, subtract them. Do not float it or shake it to guess — the leaflet and the counter are the only reliable sources, and many say to discard the inhaler at the labelled count.
The inhaler days calculator gives you the last full day and a reorder date. If you use tablets as well, the refill date calculator does the same job for pill counts. Doz does not count puffs, but it can remind you when a scheduled dose is due and nudge you again if it stays unlogged.
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