How long a bag of dog food lasts and how many to buy

How to work out how long a bag of dog food lasts

A bag lasts its weight divided by what your pet eats in a day: a 30 lb bag (480 oz) at 10 oz a day lasts 48 days. The calculator starts from the amount you enter, the one in the feeding chart on the bag or the one your vet gave you. It does not work out a portion from your pet's weight: the right amount depends on the brand, age, activity and whether the pet is spayed or neutered, and the manufacturer's chart is already made for that food.

With several pets eating from the same bag, the amount is multiplied by their number. For the supply, one day's use is multiplied by the days in the period (a month is 365.25 ÷ 12 = 30.44 days, a week is 7) and divided by the bag size, rounding up to whole bags. The calculator also tells you how many days the bags you buy will cover and how much is left over.

For a cat, litter counts too. Enter how much litter each cat uses over a number of days: what you pour in at each full change and how often you change it, or what you go through in a week. Pick whether your litter is sold by weight or by volume; the math is the same as for the food, with litter bags instead of food bags.

A worked example

In the US. The calculator opens on one dog eating 10 oz of dry food a day, 30 lb bags and a 3-month supply. 10 oz (283 g) sits between the 44 lb and 49 lb rows of the Royal Canin Medium Adult chart (271 and 291 g).

• Days: 3 × 30.44 = 91 days
• Dry food: 10 oz × 91.31 = 913 oz, 57.1 lb
• Bags: 57.1 ÷ 30 = 1.9, so 2 bags of 30 lb
• One bag lasts 480 ÷ 10 = 48 days; the two bags cover 96 days, with 2.9 lb left over

An open bag stays fresh for up to 6 weeks, 42 days, if it is closed and stored properly, so the largest bag this dog finishes in time is 26.3 lb. With 17 lb bags you need 4, lasting 27 days each. In cups: with this kibble 340 kcal per cup ÷ 3,616 kcal per kg = 0.094 kg, so one cup weighs 94 g (3.3 oz) and 10 oz is about 3 cups a day. A pound of this kibble is 453.6 ÷ 94 = about 4.8 cups; a lighter or denser kibble changes it, so check the kcal per cup on your bag.

For a cat eating 2 oz a day, a 7 lb bag lasts 56 days; 3 lb bags last 24 days and you need 4 for 3 months. With clumping litter at 5 lb a week per cat, 3 months take 65.2 lb: 4 bags of 20 lb, 28 days each.

In the UK. The same Royal Canin chart gives 271 g a day for a 20 kg dog. In 3 months that is 24.7 kg: 2 bags of 15 kg, 55 days each, with 5.3 kg left over. The chart also gives 2 7/8 cups, and the calculator shows the same figure with a 94 g cup.

How many days a bag of dog food lasts

Whole days for one pet, worked out as bag weight ÷ daily amount.

Amount a day	4 lb bag	17 lb bag	30 lb bag
4 oz	16	68	120
8 oz	8	34	60
12 oz	5	22	40
16 oz	4	17	30

In metric bags:

Amount a day	4 kg bag	10 kg bag	15 kg bag
100 g	40	100	150
200 g	20	50	75
300 g	13	33	50
400 g	10	25	37

With two pets eating from the same bag, halve the days.

Bag sizes, open bags, cups and cat litter

In the US, Royal Canin sells this dog food in 4, 17, 30 and 40 lb bags and the indoor cat food in 3, 7 and 15 lb bags; in the UK the dog food comes in 3 and 15 kg bags. Once a bag is open, Royal Canin says the kibble is at its best for a month and fine for up to 6 weeks, kept in the original bag inside an airtight container. The calculator warns you when a bag lasts more than 42 days and tells you the largest bag worth buying; you can change the limit in the advanced options.

Feeding charts in the US give cups, but a cup of kibble does not always weigh the same. Divide the kcal per cup by the kcal per kg on the bag: 340 ÷ 3,616 gives 94 g for this dog food, 325 ÷ 3,534 gives 92 g for the cat food. Royal Canin itself says grams are the most accurate measure and cups are an approximation, so weigh the portion on a kitchen scale at least once.

US litter bags are labeled by weight. With clumping litter you scoop the clumps every day and top up, so usage is easiest to measure by the week. As a reference, PetSafe says one bag of its crystal litter replaces 40 lb of clumping clay over 2 months with one cat, about 4.6 lb a week; the calculator opens on 5 lb. Non-clumping litter such as Catsan Hygiene goes in about 5 cm (2 in) deep and is changed completely at least once a week: in a 50 × 40 cm tray that is 10 L a week.

Common mistakes

• Using the amount for a different food. Kibbles have different calories, so 10 oz of one brand is not 10 oz of another. Use the chart on the bag you buy.
• Entering the amount for all your pets together. Enter it for one pet; the number of pets multiplies it.
• Reading the wrong row. Royal Canin's charts go by the pet's ideal weight, not its current weight; if in doubt, ask your vet.
• Buying the biggest bag for a small dog. If it takes more than 6 weeks to finish an open bag, the last of it is past its best. Two smaller bags open one after the other.
• Counting only the litter you top up. A full change uses litter too: if you replace it all once a month, add that amount to the month's use.