How long does a bag of dog food last? Calculator
Get your pet’s exact daily portion
Open the calculator →By Josh · Reviewed and updated: 2026-09-17
For the same bag in volume rather than days, see how many quarts in a pound of dog food.
Buying by the month instead of the bag? Monthly food amounts turns the same arithmetic into a shopping figure.
And for what a single cup is worth in energy, see calories per cup of dog food.
Want the same bag measured in scoops instead of days? See how many cups are in a bag of dog food.
A 30 lb bag of dog food lasts a 40 lb dog roughly 47-50 days at typical feeding rates (exact days depend on the food's calorie density). The chart below gives the duration for any bag size and dog weight, calculated from NRC calorie equations rather than the bag's overfeeding guidelines.
No kitchen scale? How to measure dog food without a scale covers cups, hands and jars.
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
Keep it fresh: a big bag opened today is still good in week 5 only if it is sealed well — an airtight storage container keeps the last weeks as good as the first.
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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.
If you measure your daily amount in cups rather than grams, convert it first. A cup of dry food weighs anywhere from about 90 to 130 grams depending on the kibble, so the conversion is specific to your food — the cups and pounds reference covers the range and includes a converter.
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, freshness and shelf life
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.
That window is the mix’s shelf life once the bag is open — which means a bag that takes much longer than that typical six-week window to finish is likely past its freshness window before it is past its calorie budget.
The practical move is to buy the size that fits the window rather than the size that is cheapest per kilogram — dry dog food bag sizes are listed by weight on Amazon, and a 22 lb (10 kg) bag is a common fit for a medium dog eating around 200 g a day.
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.
Itchy dog year-round? The culprit is usually protein, not grain — our dog food allergy guide ranks the real triggers from a 297-dog study.
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.
How long does dog food last once the bag is open?
Around six weeks is the figure that appears most often for an opened bag, and your own bag is the one that applies. Dry food is not shelf-stable once air is in the bag — the fats oxidise, the aroma compounds fade and the vitamin premix degrades. How long your bag lasts is the calculation above; how long it stays good is this.
Calculate your pet's daily food amount →
Work out the cost per day →
How many treats can your dog have? →
Storing it: which container, and how big a bag →
Save on flea, tick and heartworm meds →
Switching formulas? See the calorie math of switching dog food safely — why the same cups are not the same calories.
Comparing wet and dry food? See wet vs dry dog food — calorie density, water content and cost compared.
Wondering about freshness after opening? See how long dry dog food actually lasts — the 4-6 week window and storage rules.
Where the numbers come from
Every figure on this page is arithmetic on two values you can read off your own bag, not an estimate of how much a dog “should” eat. Daily intake starts from resting energy (RER = 70 × kg0.75), is multiplied by a life-stage factor, and is then divided by the food’s metabolisable energy to give grams per day. Days per bag is simply the net bag weight divided by that daily gram figure.
Two limits are worth stating plainly. First, the life-stage factors are population averages — an individual dog can sit 20–30% either side of the calculated number and still be in normal condition. Second, a bag’s usable life is not the same as the date printed on it: once the bag is opened the clock changes, which is covered separately in how long dry dog food actually lasts.
How this page was checked: Numbers traced to NRC (2006), WSAVA and AAHA guidance; units re-derived independently; safety statements limited to the species they were established for. Our review method
Primary references
- MSD Veterinary Manual — Nutrition, Small Animals. Resting and maintenance energy requirements, used here as the denominator. msdvetmanual.com
- AAFCO — consumer resources on pet food labels. Where the metabolisable energy (kcal/kg) statement on a bag comes from and what it is required to state. aafco.org/consumers
- Tufts University, Cummings School of Veterinary Medicine — Petfoodology. Fat oxidation and rancidity: why an opened bag degrades faster than the printed date implies. sites.tufts.edu/petfoodology
- AAHA — nutritional assessment guidelines for dogs and cats. Body condition scoring, the check that tells you whether the calculated amount is actually right. aaha.org