Running out of pet food on a Sunday evening is a small crisis that a little arithmetic prevents. The sum is easy: how much is in the bag, how much goes into the bowl each day, and how long delivery takes. This guide turns that into a routine, with a formula, a worked example, a table of how long bags last at common daily amounts and a short setup for reminders and subscriptions. It also covers what to do when the daily amount changes, which is the most common reason the sums go wrong. The Pet Food Run-Out Date Calculator does the date arithmetic.
The three formulas
days a bag lasts = bag grams ÷ grams per day run-out date = date opened + days reorder date = run-out date − delivery lead time − buffer
The first is the one that matters. Our guide to how long a bag of dog food lasts explains it in detail, and the Pet Food Bag Duration Calculator gives the days.
Worked example
A neutered 20 kg dog needs about 1,059 kcal a day. The food is 3,600 kcal per kg, which is 3.6 kcal per gram. Daily grams = 1,059 ÷ 3.6 = 294 g.
| Step | Working | Result |
|---|---|---|
| Bag size | 10 kg | 10,000 g |
| Days the bag lasts | 10,000 ÷ 294 | 34 days (34.0) |
| Opened on | 19 October 2026 | |
| Run-out date | 19 Oct + 34 | 22 November 2026 |
| Delivery lead time | 5 days | |
| Reorder by | 22 Nov − 5 | 17 November 2026 |
If you want a buffer of two more days, order by 15 November. The calendar entry is the only thing to set.
How long bags last
| Daily grams | 2 kg bag | 5 kg bag | 10 kg bag | 15 kg bag |
|---|---|---|---|---|
| 60 g (small cat) | 33 days | 83 days | 167 days | 250 days |
| 100 g | 20 | 50 | 100 | 150 |
| 200 g | 10 | 25 | 50 | 75 |
| 294 g | 6.8 | 17 | 34 | 51 |
| 400 g | 5 | 12.5 | 25 | 37.5 |
Round down to whole days for planning. A 10 kg bag lasting 34 days at 294 g a day means a monthly rhythm, but not exactly one month: 34 days is longer than a typical month, and a reminder tied to the calendar month will drift.
Setting a lead time and buffer
| Source of food | Typical lead time | Buffer to keep |
|---|---|---|
| Local shop | 0 to 1 day | 1 to 2 days |
| Online delivery | 2 to 5 days | 3 to 5 days |
| Subscription | 3 to 7 days | 5 to 7 days |
| Prescription or special diet | May take longer; ask the supplier | A week or more |
Choose a lead time from your own experience: the longest delivery in the last few months, not the average. Prescription diets and unusual brands may be out of stock; if so, keep a larger buffer. If the shop has a stock-out, your buffer is what stops you switching food suddenly; see how to switch dog food for why a sudden change is best avoided.
Count treats and toppers
If you want to be precise, your daily grams include only the meal food. Treats come from the same calorie budget (up to 10% of the day, see how many treats a dog can have), so a bigger treat allowance means a smaller meal. For planning, it is simplest to calculate meal grams after treats are subtracted.
| Daily energy | Treats (10%) | Meal energy | Meal grams at 3.6 kcal/g |
|---|---|---|---|
| 1,059 kcal | 106 kcal | 953 kcal | 265 g |
At 265 g a day the 10 kg bag lasts 37.7 days, not 34. A difference of about three days is enough to matter for a reminder.
When the daily amount changes
Every change in the daily amount shifts the run-out date. Typical reasons:
- A weight-loss or weight-gain plan
- A change of food, which changes the kcal per gram
- A season, a change in activity or a life stage
- A puppy or kitten growing
- A second pet sharing the bag
Recalculate when it happens. If you have used half the bag at 294 g a day and then move to 265 g a day, the rest of the bag lasts longer.
| Step | Working |
|---|---|
| Bag opened on 19 Oct, used for 17 days at 294 g | 4,998 g used |
| Remaining | 10,000 − 4,998 = 5,002 g |
| New daily amount | 265 g |
| Days left | 5,002 ÷ 265 = 18.9 |
| New run-out date | 5 Nov + 18 days = 23 November 2026 |
The run-out date moves from 22 to 23 November: one day. The Pet Food Run-Out Date Calculator does this for any opening date and daily amount.
Setting up reminders
- Work out the days a bag lasts and write it on the bag with a marker.
- Put the run-out date in your calendar on the day you open the bag.
- Set a second alert on the reorder date.
- Make it repeat at the days-per-bag interval if the daily amount is stable.
- Review after each bag or after each change in food or weight.
Many shops offer subscriptions with a chosen interval. Set the interval to the days per bag, minus a margin, and check that the first delivery arrives before the old bag is empty. A subscription is convenient, but it does not know when your pet’s amount changes, so review it.
Keeping a simple order log
| Date opened | Bag size | Daily grams | Days | Run-out date | Reorder by | Ordered on | Arrived on |
|---|---|---|---|---|---|---|---|
| 19 Oct 2026 | 10 kg | 294 | 34 | 22 Nov | 17 Nov |
The last two columns tell you how long deliveries really take. After three or four rows, you know your true lead time.
Using the log to compare prices
When you record the price of each bag, you can compare cost per day and cost per 1,000 kcal; see price per 1,000 kcal. A bag at 36.00 lasting 34 days costs 1.06 a day. If you travel, our guide on how much pet food to pack for a trip covers the extra amounts, and the Pet Travel Food Calculator handles them.
A worked example
Lena’s cat eats 52 g a day from a 2 kg bag. 2,000 ÷ 52 = 38.5, so the bag lasts 38 days. She opens it on 1 March, so it runs out on 8 April. Her online shop delivers in 3 to 4 days, and she wants a three-day buffer, so she reorders by 8 April − 4 − 3 = 1 April. She puts the date in her phone, ordering on 31 March to be safe. The bag arrives on 3 April and is stored in a sealed container. Lena also notes the vet may change the amount at the next check-up, and plans to redo the sum then.
Two pets, one bag
When two pets share a bag, add their daily amounts. A 20 kg dog on 294 g a day and a second dog on 150 g a day use 444 g together.
| Bag | Daily total | Days it lasts | Opened 19 Oct 2026, runs out |
|---|---|---|---|
| 10 kg | 444 g | 22.5 (plan 22) | 10 November 2026 |
| 15 kg | 444 g | 33.8 (plan 33) | 21 November 2026 |
(19 October plus 22 days is 10 November; plus 33 days is 21 November.) Round the days down for the reminder. If the two dogs eat different foods, keep a separate row in the log for each bag, so one dog’s change of food does not hide in the other’s date. A bigger bag lasts longer but only helps if you can store it sealed and use it within the best-before date printed on the pack.
Common mistakes
- Setting a monthly reminder for a 34-day bag.
- Using the label’s feeding table instead of the real daily amount.
- Ignoring treats.
- Forgetting to update the date after a change.
- Using the average delivery time instead of the longest.
- Leaving no buffer for out-of-stock items.
- Not rechecking a subscription interval.
Quick summary
- Days = bag grams ÷ grams per day; run-out date = open date + days.
- Reorder date = run-out date − lead time − buffer.
- A 10 kg bag at 294 g a day lasts 34 days; opened 19 October 2026 it runs out on 22 November, so reorder by 17 November with a 5-day lead.
- Recalculate when the daily amount, food or weight changes.
- Log each bag to learn your real lead time.
- Use the Pet Food Run-Out Date Calculator.
Frequently asked questions
How do I know when my pet's food will run out?
Divide the bag's grams by the daily grams, and add the days to the date you opened the bag.
When should I reorder?
Subtract your delivery lead time and a small buffer from the run-out date.
Should I include treats?
If you want accuracy, yes. Count treats in the daily total or keep them as a separate item.
What if my pet's weight changes?
Recalculate the daily amount and the run-out date whenever the amount changes.
How big a buffer should I keep?
A few days is common. Choose a number that covers a late delivery.
Can I set a repeating reminder?
Yes. Once you know the days per bag, set a repeating reminder in your calendar or the shop's subscription.
Sources
- Pure arithmetic: days = bag grams ÷ daily grams; run-out date = open date + days; reorder date = run-out − lead time − buffer
- Daily grams from NRC (2006) energy factors via the Dog Calorie Calculator (neutered 20 kg adult: 1,059 kcal a day)
Related calculators
- Pet Food Run-Out Date CalculatorFind the date a bag of food will run out and when to reorder.
- Pet Food Bag Duration CalculatorFind out how many days a bag of pet food will last at the amount you feed.
- Pet Travel Food CalculatorWork out how much pet food to pack for a trip, with spare days.
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.
- How Much Pet Food to Pack for a TripHow much pet food to pack: (trip days + spare days) × daily grams × pets, with tables for a dog and a cat, spare-day rules, wet and dry food and a packing checklist.
- 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.