How long does a bag of dog food last?
By Josh · Reviewed and updated: 2026-09-17
The bag does not tell you how long it will last, and the feeding chart on the back is a population average — so it will not tell you either. What you need is your pet's actual daily amount in grams. That takes about thirty seconds to work out, and then the bag duration is simple division.
Bag duration calculator
How the calculation works
There is no hidden maths in a bag duration estimate. It is two steps:
- Work out the daily amount in grams, from your pet's weight, life stage and activity level together with the calorie content of the food.
- Divide the weight of the bag by that figure.
The reason bag duration is worth calculating rather than guessing is step 1. Two dogs of the same weight can need very different amounts, because a neutered adult on a typical routine needs roughly 11% fewer calories than an intact dog of the same size, and a working dog needs over half again as much as the same dog on a typical routine. The chart on the bag cannot know any of that.
Calorie density matters just as much. The same dog eating a 370 kcal/cup food needs noticeably fewer grams per day than when eating a 320 kcal/cup food, so the same size bag lasts longer.
Reference table: days per bag, by bag size and dog weight
Calculated for a neutered adult dog on a typical activity level, eating a dry food of 370 kcal/cup and 110 g per cup. Rounded to the nearest day. Use the calculator above for your own numbers — particularly if your dog is intact, very active, a puppy, or on a diet.
| Dog's weight | 5 lb bag | 15 lb bag | 30 lb bag | 40 lb bag |
|---|---|---|---|---|
| 5 kg — 11 lb | 20 days | 61 days | 122 days | 163 days |
| 10 kg — 22 lb | 12 days | 36 days | 73 days | 97 days |
| 20 kg — 44 lb | 7 days | 22 days | 43 days | 58 days |
| 30 kg — 66 lb | 5 days | 16 days | 32 days | 43 days |
| 40 kg — 88 lb | 4 days | 13 days | 26 days | 34 days |
Read the table from the bottom up and the small-bag problem becomes obvious. A 40 kg dog empties a 5 lb bag in about four days, which is a lot of packaging per meal. A 5 kg dog takes over five months to get through a 40 lb bag — which is the more serious problem of the two, because it brings us to freshness.
Bulk buying and freshness
Larger bags are cheaper per kilogram, which is why most owners buy them. But dry food is not shelf-stable once it is open. The fats in it oxidise, the aroma compounds that make it palatable fade, and the vitamin premix degrades over time. All three happen faster once air is in the bag.
Most manufacturers print a “use within” window somewhere on the packaging, and around six weeks is the figure that appears most often for an opened bag. Check your own bag — that is the number that applies to your food. The calculator above flags it when a bag will be open longer than six weeks, so you can check it against whatever your manufacturer says.
This is the real trade-off when choosing a bag size, and it is a case where the cheaper option can be the false economy:
- If your pet finishes the bag inside the freshness window, buy the largest bag you can — you are just paying less per kilogram for the same product.
- If it will not, a slightly smaller bag that you finish while it is still fresh is worth more than the per-kilogram saving on a bag that goes stale in the cupboard.
- If you buy in bulk anyway, keep the food in the original bag, inside an airtight container, and do not tip it out. The bag's own lining is part of the packaging design, and mixing an old bag into a new one means the old food never gets finished.
Where to keep it
Heat, light and moisture are what do the damage, so a cool dark cupboard beats a sunny shelf or a warm utility room. Keeping the bag sealed is more important than keeping it tidy.
Two specific mistakes worth avoiding: storing dry food next to a boiler or in a garage that swings between extremes of temperature, and pouring a new bag on top of the remains of an old one. The second one means you are always feeding the oldest food in the house.
Cats
The arithmetic is identical — cats just empty a bag more slowly, which makes the freshness trade-off sharper. A typical 4 kg neutered indoor cat eats far less by weight than a dog of the same size, so a large bag of dry cat food can easily be open for several months in a single-cat household. For most single cats, a smaller bag is the better buy even at a higher price per kilogram.
Frequently asked questions
How long does a 30 lb bag of dog food last?
It depends almost entirely on the size of the dog. For a neutered adult dog on a typical activity level eating a 370 kcal/cup food, a 30 lb bag lasts roughly 73 days for a 10 kg dog, 43 days for a 20 kg dog, 32 days for a 30 kg dog, and 26 days for a 40 kg dog. Use the calculator above for your dog's exact figure.
Does a bigger bag really work out cheaper?
It is cheaper per kilogram, but that is not the same as cheaper per day, and it is not cheaper at all if the food goes stale before it is eaten. Work out the days per bag first, compare that with the use-by guidance on your packaging, and only then decide whether the larger bag is worth it.
Should I keep dry food in the original bag or a storage bin?
Keep it in the original bag and put the whole bag inside an airtight container. The bag's lining is designed to hold the food's fats away from air. Tipping the food out into a bin exposes it to more air and to the oils left behind from previous batches.
How much food should I keep in the bowl or feeder?
Only a few days' worth. Topping up a feeder with a fortnight of food at a time means the food at the bottom sits exposed for that whole period. Refilling a dispenser more often, with less each time, keeps the food fresher for the same effort.
Calculate your pet's daily food amount →
Work out the cost per day →
How many treats can your dog have? →