The guaranteed analysis on a pet food label has four required items: crude protein, crude fat, crude fibre and moisture. Ash is not among them. Yet the usual way of estimating calories needs it, because carbohydrate is not listed at all and is found by subtracting everything else from 100. When ash is missing, the sum has a gap, and most guides fill it with a typical number. This guide explains why ash matters, which typical numbers to use, how much the guess changes the answer for a dry and a wet food and what to do about it. The short version is that the effect is a few percent, which is smaller than other errors in the sum, but worth knowing. The Dog Food Calorie Calculator uses defaults of 8% ash for dry and 2.5% for wet food when you leave it blank.
Why ash enters the sum
Calories come from protein, fat and carbohydrate. The label gives protein and fat, but carbohydrate has to be inferred.
carbohydrate (NFE) = 100 − protein − fat − fibre − moisture − ash
Whatever is not accounted for by the other parts must be carbohydrate (nitrogen-free extract). So each percentage point of ash that you assume takes one point away from carbohydrate and, at 3.5 kcal per gram, removes 3.5 kcal from each 100 g of food.
kcal per 100 g = 3.5 × protein + 8.5 × fat + 3.5 × NFE
A dry food: ash from 6 to 10%
An example dry food has protein 26, fat 14, fibre 4 and moisture 10.
| Assumed ash | NFE | Protein | Fat | NFE | kcal per 100 g | kcal per kg |
|---|---|---|---|---|---|---|
| 6% | 40 | 91.0 | 119.0 | 140.0 | 350.0 | 3,500 |
| 8% (default) | 38 | 91.0 | 119.0 | 133.0 | 343.0 | 3,430 |
| 10% | 36 | 91.0 | 119.0 | 126.0 | 336.0 | 3,360 |
The spread from 6% to 10% ash is 14 kcal per 100 g, which is 4.0% of the middle value. Each point of ash is 1.0% of the energy. A 20 kg dog needing 1,059 kcal would be fed 303 g at 3,500 kcal/kg, 309 g at 3,430 and 315 g at 3,360: a range of 12 g, or about 4%.
A wet food: ash from 1.5 to 3.5%
An example wet food has protein 8, fat 5, fibre 1 and moisture 78.
| Assumed ash | NFE | Protein | Fat | NFE | kcal per 100 g | kcal per kg |
|---|---|---|---|---|---|---|
| 1.5% | 6.5 | 28.0 | 42.5 | 22.75 | 93.25 | 933 |
| 2.5% (default) | 5.5 | 28.0 | 42.5 | 19.25 | 89.75 | 898 |
| 3.5% | 4.5 | 28.0 | 42.5 | 15.75 | 86.25 | 863 |
For the wet food the spread over the same two-point range is 7.0 kcal per 100 g, 7.8% of the middle value. The relative effect is larger because the total energy is small, so each point of carbohydrate is a bigger share. A cat needing 238 kcal would get 255 g of the first, 265 g of the second and 276 g of the third.
Comparing the two
| Dry | Wet | |
|---|---|---|
| Default ash | 8% | 2.5% |
| Ash range tested | 6 to 10% | 1.5 to 3.5% |
| Energy range | 336 to 350 kcal/100 g | 86 to 93 kcal/100 g |
| Spread as % of middle | 4.1% | 7.8% |
| Effect on a day’s food | about ±2% | about ±4% |
Both are small compared with the 10% or more by which an Atwater estimate can differ from a measured value, but they add up.
Better than a default
If you can get a real number, use it.
- Check the pack. Some foods list ash, and some list it as “crude ash” or “minerals”.
- Check the maker’s website. Many publish typical analyses with ash.
- Ask the maker. A short email asking for the ash value or the metabolizable energy often gets an answer within days.
- Use the calorie statement. If the pack gives kcal per kg, use that instead of an estimate.
A stated calorie value from the maker is generally better than any calculation from the guaranteed analysis, though it may itself be calculated.
Checking by the scales
The most reliable check is the pet. Feed the estimated amount, weigh every few weeks and adjust. If the estimate was 4% off, the weight will drift by an amount you can see in a month or two. This is the same method as in how to track your pet’s weight change. A calorie estimate should be treated as a starting point, and a margin of 10% is common.
Dry matter and ash
Ash also affects the dry-matter comparison of nutrients. Dry matter is 100 minus moisture, so ash does not change it directly; but the minerals it represents are part of what the food is made of. See dry matter basis explained and the Dry Matter Basis Calculator for the conversion. For a cat food, the same method applies, as in how to read a cat food label.
A worked example
Lena has a bag of dry dog food that lists protein 24, fat 13, fibre 3.5 and moisture 10. Ash is not listed. She uses 8% and also tries 6% and 10%. Her dog needs 1,059 kcal.
| Ash | NFE | kcal per 100 g | kcal per kg | Grams for 1,059 kcal |
|---|---|---|---|---|
| 6% | 43.5 | 84.0 + 110.5 + 152.25 = 346.75 | 3,468 | 305 |
| 8% | 41.5 | 84.0 + 110.5 + 145.25 = 339.75 | 3,398 | 312 |
| 10% | 39.5 | 84.0 + 110.5 + 138.25 = 332.75 | 3,328 | 318 |
The range is 13 g, 4% of the daily amount. Lena feeds 312 g, weighs the dog every two weeks and emails the maker for the real figure. When the maker replies with ME 3,380 kcal/kg, she changes her daily amount to 313 g, a change of 1 g.
Asking the maker
An email is often the quickest way to a real figure. Something like:
Hello. I feed [product name, batch or best-before date] to my dog. The label does not list ash or metabolizable energy. Could you tell me the typical ash content and the metabolizable energy in kcal per kg, and how it was determined? Thank you.
Include the product name and the code on the bag. Makers usually reply with a figure or a data sheet. If the answer is a calculated figure, ask for the factors used. Record the reply with the date.
What the guess does to the bill
The ash guess also moves the grams and so the cost. For the dry example at 3.60 per kg and a dog needing 1,059 kcal:
| Ash assumed | kcal/kg | Grams a day | Cost a day |
|---|---|---|---|
| 6% | 3,500 | 303 | 1.09 |
| 8% | 3,430 | 309 | 1.11 |
| 10% | 3,360 | 315 | 1.13 |
A difference of 0.04 a day is small. The bigger risk is not the cost, but the dog gaining or losing weight if the guess is far off. Weigh the dog every few weeks, and adjust.
Cat foods and treats
The same method applies to cat food and treats, with the same limits. For a cat, the amounts are small, so a 4% error in the energy is a few grams a day, which is noticeable on a scale that reads to 1 g. Treats often have no ash figure; use the dry default and keep a margin, as in treat calories per treat. For foods with a lot of bone or mineral content, the default may understate ash, so ask the maker.
Common mistakes
- Treating a calorie estimate as exact.
- Forgetting to subtract ash and fibre when finding carbohydrate.
- Using a dry default on a wet food.
- Not asking the maker for the real figure.
- Ignoring the effect on a small pet.
- Not checking with the scales.
- Mixing up crude fibre and total fibre.
Quick summary
- Carbohydrate = 100 − protein − fat − fibre − moisture − ash; kcal = 3.5 × protein + 8.5 × fat + 3.5 × NFE per 100 g.
- When ash is missing, assume about 8% for dry and 2.5% for wet.
- A 4-point swing in ash moves a dry food by about 4% (336 to 350 kcal per 100 g).
- For a wet food a 2-point swing moves the estimate about 8% (86 to 93 kcal per 100 g).
- Ask the maker for the actual figure, and check against your pet’s weight.
- Use the Dog Food Calorie Calculator.
Frequently asked questions
What is ash on a pet food label?
The mineral residue left after the food is burned. It is part of the food that provides no energy.
What if the label has no ash figure?
Use a typical value (about 8% for dry, 2.5% for wet) in the calorie estimate and treat the result as approximate.
How much does the ash guess matter?
A swing of 2 points either way moves a dry food by about 2% and a wet food by about 8%, because each point of ash is about 1% of a dry food's energy and about 4% of a wet food's.
Can I get the actual figure?
Yes. Ask the maker for the ash value or the metabolizable energy. Many will tell you.
Is the estimate good enough to feed by?
As a starting point, yes. Check the result against your pet's weight and adjust.
Why does ash change the carbohydrate figure?
Carbohydrate is found by subtracting everything else from 100. If ash is higher, carbohydrate is lower.
Sources
- NRC (2006) Nutrient Requirements of Dogs and Cats, National Academies Press: modified Atwater factors 3.5 kcal/g protein, 8.5 kcal/g fat, 3.5 kcal/g carbohydrate
- AAFCO Pet Food Label Guide: guaranteed analysis lists crude protein, crude fat, crude fibre and moisture; ash is not required on the label
Related calculators
- Dog Food Calorie CalculatorWork out how many calories are in a dog or cat food from the nutrient percentages (guaranteed analysis) on its label, when no kcal figure is printed.
- Dry Matter Basis CalculatorCompare wet and dry pet foods fairly by removing the water from the label percentages.
- Pet Food Energy ConverterConvert pet food energy between kcal per kg, per 100 g, per pound and per ounce.
Keep reading
- Calculate Calories in Dog Food From the LabelHow to estimate calories in dog food from the guaranteed analysis using modified Atwater factors: dry and wet worked examples, NFE and the limits.
- Dry Matter Basis: Compare Wet and Dry FoodLearn how dry matter basis works, the formula to convert any pet food label, worked examples for wet, dry and semi-moist foods and common label traps.
- How to Read a Cat Food LabelHow to read a cat food label: guaranteed analysis, moisture, dry-matter and Atwater calorie estimates, a worked dry and wet comparison and the feeding chart.
- Pet Food Energy Units: kcal, kJ and MJHow to convert pet food energy between kcal, kJ and MJ: 1 kcal = 4.184 kJ, worked examples for a 1,059 kcal dog and a 15 MJ/kg food, and what label Calories mean.