The price tag on a large bag looks like a bargain, and often it is. The saving per kilogram can be 20 to 30% between the smallest and largest sizes. But a bargain you cannot finish is not a bargain. Dry food is sold with a best-before date and instructions for storage after opening, and a bag that is open for too long may need to be thrown away or fed past the point the maker recommends. This guide turns the question into arithmetic: how much each bag saves per day, per month and per year, how a use-within limit changes the answer and how to find the largest bag worth buying for your pet. The prices are examples. The Pet Food Bag Duration Calculator gives the days a bag lasts.
Cost per day by bag size
A 20 kg neutered dog eats 294 g a day of a 3,600 kcal/kg food (see dog food cost per day by weight). The example bags are:
| Bag | Price | Price per kg | Days it lasts | Cost per day | Per month | Per year |
|---|---|---|---|---|---|---|
| 2 kg | 9.00 | 4.50 | 6.8 | 1.32 | 40.2 | 483 |
| 5 kg | 20.00 | 4.00 | 17.0 | 1.18 | 35.7 | 429 |
| 10 kg | 36.00 | 3.60 | 34.0 | 1.06 | 32.2 | 386 |
| 15 kg | 49.50 | 3.30 | 51.0 | 0.97 | 29.5 | 354 |
(Cost per day = 0.294 kg × price per kg. A month is 30.4 days.) The saving from the smallest to the largest bag is 129 a year, about 27%. On the face of it, the 15 kg bag wins.
The catch: time after opening
Dry food has a best-before date on the pack. After opening, makers usually give storage instructions, and some give a time within which to use it. Follow what is printed on your pack. For this guide take an example limit of 42 days from opening.
maximum useful bag (kg) = daily grams × use-within days ÷ 1,000
For 294 g a day and 42 days: 0.294 × 42 = 12.3 kg. A bag larger than that cannot be finished in time.
| Bag | Days to finish | Within 42 days? | Eaten | Wasted | Effective price per kg eaten |
|---|---|---|---|---|---|
| 2 kg | 6.8 | Yes | 2.00 kg | 0 | 4.50 |
| 5 kg | 17.0 | Yes | 5.00 kg | 0 | 4.00 |
| 10 kg | 34.0 | Yes | 10.00 kg | 0 | 3.60 |
| 15 kg | 51.0 | No | 12.35 kg | 2.65 kg (17.7%) | 4.01 |
The 15 kg bag’s effective price is 49.50 ÷ 12.35 = 4.01 per kg eaten, which is dearer than the 10 kg bag at 3.60. The 10 kg bag is the best choice for this dog, and it saves 0.26 a day against the 2 kg bag. The extra five kilograms saved nothing and carried a risk of waste.
The same dog in a two-dog home
Add a second dog eating 150 g a day, for a combined 444 g. Now the limit is 0.444 × 42 = 18.6 kg, and the 15 kg bag lasts 33.8 days.
| Bag | Days to finish | Within 42 days? | Cost per day (444 g) |
|---|---|---|---|
| 10 kg | 22.5 | Yes | 1.60 |
| 15 kg | 33.8 | Yes | 1.47 |
With more mouths, the larger bag is the cheaper choice and saves 0.13 a day (49 a year). The right bag depends on your own daily use.
Cash up front
A bigger bag takes more money at once. The saving is spread over weeks, and the cost is paid at the till.
| Bag | Cash at purchase | Saving per year versus 2 kg | Saving as % of cash |
|---|---|---|---|
| 5 kg | 20.00 | 54 | 270% |
| 10 kg | 36.00 | 97 | 269% |
| 15 kg | 49.50 | 129 (if finished) | 261% |
(Annual saving, relative to the 2 kg price of 4.50 per kg.) If cash is tight, a mid-sized bag is a reasonable compromise: the 10 kg bag gets 75% of the available saving for 73% of the 15 kg bag’s cost.
Storage and freshness
A large bag needs the right storage. Follow the pack’s instructions: keep it sealed, dry and out of heat. Some owners split a large bag into smaller sealed containers so that only one is open at a time, though you should keep the information from the original pack (the batch and best-before date). This is general practice; the manufacturer’s guidance for the product is the rule. Our guide on reorder reminders explains how to set the date to reorder.
Does cost per kg tell the whole story?
Not quite. Compare foods by cost per 1,000 kcal if the bags are different foods; see price per 1,000 kcal. If you compare bags of the same food, price per kg is enough. Also include delivery: a delivery fee of 5.00 on a 2 kg bag adds 2.50 per kg, which is more than the whole saving from a bigger bag.
| Bag | Price | Delivery | Price per kg delivered |
|---|---|---|---|
| 2 kg | 9.00 | 5.00 | 7.00 |
| 10 kg | 36.00 | 5.00 | 4.10 |
| 15 kg | 49.50 | 5.00 | 3.63 |
Free delivery above a threshold can change the choice: a 10 kg bag might qualify when a 5 kg bag does not.
A worked example
Mei has a cat eating 52 g of dry food a day. She is choosing between a 2 kg bag at 11.00 and a 6 kg bag at 27.00. The pack says to use within 8 weeks (56 days) of opening.
| Step | Working | Result |
|---|---|---|
| 2 kg bag lasts | 2,000 ÷ 52 | 38.5 days |
| 6 kg bag lasts | 6,000 ÷ 52 | 115 days |
| Largest useful bag | 0.052 × 56 | 2.9 kg |
| 6 kg bag eaten in 56 days | 52 × 56 = 2,912 g | 2.9 kg |
| Wasted | 6.0 − 2.9 | 3.1 kg (52%) |
| Effective price of 6 kg bag | 27.00 ÷ 2.9 | 9.31 per kg |
| Price of 2 kg bag | 11.00 ÷ 2 | 5.50 per kg |
For one small cat, the large bag looks cheaper at 4.50 per kg but costs 9.31 per kg eaten because over half would be wasted. Mei buys the 2 kg bag. If she had three cats, the sum would change, and so would the answer.
The largest useful bag for different pets
Using the same 42-day example limit, the biggest bag worth buying depends only on how much is eaten each day.
| Daily amount | Largest useful bag (kg) | Example |
|---|---|---|
| 52 g | 2.2 | A 4 kg cat |
| 150 g | 6.3 | A small dog |
| 294 g | 12.3 | A 20 kg dog |
| 444 g | 18.6 | Two dogs sharing a bag |
| 600 g | 25.2 | A large dog or several |
(Daily grams × 42 ÷ 1,000.) Pick the largest bag size at or below this figure that the shop sells. For the cat, a 2 kg bag is the best fit, and a 6 kg bag would run into the limit. If your pack gives a different time, or none, use what is printed and ask the maker if it is unclear.
Splitting a large bag
Some owners divide a large bag into smaller airtight containers so that only one is open at a time. That can keep a bag fresh in practice, but it does not give you permission to ignore the date on the pack, and it does not change the arithmetic of how long a bag lasts. Keep the original label, with the batch and best-before date, with the containers. If the maker’s instructions say to keep food in its original bag, follow them. The question this guide asks, whether a bigger bag saves money, is answered by the price per kilogram eaten, not the price per kilogram bought.
Common mistakes
- Comparing price per kg without checking use-within time.
- Ignoring delivery fees.
- Forgetting to add other pets’ food to the daily use.
- Buying more than can be stored properly.
- Using a bag past the date on the pack.
- Treating the cheapest per kg as the cheapest per day.
- Not recalculating when the daily amount changes.
Quick summary
- Cost per day = daily grams × price per kg ÷ 1,000; bigger bags usually lower the price per kg.
- At 294 g a day, 2, 5, 10 and 15 kg bags cost 1.32, 1.18, 1.06 and 0.97 a day.
- Largest useful bag = daily grams × use-within days ÷ 1,000; for 42 days that is 12.3 kg.
- A 15 kg bag that wastes 17.7% costs 4.01 per kg eaten, more than a 10 kg bag at 3.60.
- Include delivery and cash up front.
- Use the Pet Food Bag Duration Calculator.
Frequently asked questions
Is a bigger bag cheaper?
Usually per kilogram, but not always per kilogram eaten. If food goes unused or stale, the effective price rises.
How big a bag should I buy?
No more than daily grams × the use-within days on the pack ÷ 1,000. For 294 g a day and 42 days that is 12.3 kg.
Where do I find the use-within time?
On the pack, usually as a best-before date and storage instructions. Follow the manufacturer's guidance.
What if two pets share a bag?
Add their daily amounts. A larger combined use makes bigger bags more sensible.
Does a bigger bag need more storage care?
Yes. Keep it sealed, dry and cool as the pack says, and consider decanting part into smaller containers.
Is a subscription discount the same as a bulk discount?
No. It comes with different terms. See the subscription break-even guide.
Sources
- Pure arithmetic: days = bag grams ÷ grams per day; cost per day = grams per day × price per kg ÷ 1,000; effective price = price ÷ share eaten
- Manufacturer's best-before date and storage instructions printed on the pack (the authority for how long an opened bag should be kept)
Related calculators
- Pet Food Bag Duration CalculatorFind out how many days a bag of pet food will last at the amount you feed.
- Pet Food Price per 1,000 kcal CalculatorCompare two foods fairly by price per 1,000 calories instead of price per bag.
- Pet Food Run-Out Date CalculatorFind the date a bag of food will run out and when to reorder.
Keep reading
- How Long Does a Bag of Dog Food Last?How long a bag of dog food lasts: weigh the bag, divide by daily grams, with a table for 5 to 30 kg dogs and 2 to 15 kg bags, plus storage and reorder timing.
- Pet Food Reorder Reminder SetupHow to work out when pet food runs out and when to reorder: bag grams ÷ daily grams, a lead-time rule, a worked 10 kg bag example, a reminder table and an order log.
- Price per 1,000 kcal: Compare Pet Food FairlyWhy price per 1,000 kcal is the fair way to compare pet foods: the formula, worked dog and cat examples across dry, wet and premium foods, and cost per day.
- Pet Food Subscription: Is the Discount Worth It?How to test a pet food subscription: break-even discount = shipping ÷ price, a 36.00 bag example, interval surplus or shortfall, and the terms to read.