Energy requirement and daily ration: how the calculation actually works

The starting point is resting energy requirement, the energy an awake but inactive animal needs in a thermally neutral environment, several hours after eating. Raise body weight in kilograms to the power of 0.75 and multiply by 70. The exponent is not arbitrary: metabolism does not scale in direct proportion to mass, which is why a 5 kg cat does not burn half of what a 10 kg dog burns.

• General formula: resting requirement in kilocalories equals 70 multiplied by body weight in kilograms raised to the power of 0.75.
• Linear approximation valid from roughly 2 to 45 kg: 30 multiplied by body weight, plus 70. For a 20 kg dog that gives 670 against 662 from the exponential formula, a negligible difference.
• Outside that range, meaning very large dogs and very small animals, the linear shortcut loses accuracy and the exponential formula should be used.
• Use ideal weight, not current weight, whenever the animal is overweight: calculating on current weight simply maintains the excess.

A practical note on the exponential formula: any scientific calculator or spreadsheet handles it, and the answer should be rounded to whole numbers. Carrying decimals is pointless, because individual variation far exceeds the precision of the arithmetic. Resting requirement is a starting estimate, not a measurement.

Resting requirement is multiplied by a factor reflecting reproductive status, age, activity and goal. This is where a neutered indoor cat parts company with a working dog, and skipping this step is the commonest cause of overfeeding.

The feline figures deserve a pause, because they surprise people. A neutered 4 kg indoor cat has a very modest requirement, and most rations recommended on packaging exceed it. That is why feline obesity is so common: no dramatic excess is needed, just twenty or thirty extra kilocalories a day sustained for a couple of years.

The calculation closes by converting kilocalories into grams. You need the food's energy density, declared on the label or estimated from the analytical constituents. From there it is a single division.

1. 1

   Example one: neutered 20 kg dog, normal activity

   Resting requirement: 70 times 20 raised to 0.75, about 662 kilocalories. Factor 1.6, so around 1060 kilocalories a day. With a kibble at 3570 kilocalories per kilogram, meaning 3.57 per gram, the ration is about 297 grams a day, split into two meals.

2. 2

   Example two: neutered 4.5 kg indoor cat

   Resting requirement: 70 times 4.5 raised to 0.75, about 216 kilocalories. Factor 1.2, so around 260 kilocalories a day. With a wet food at 80 kilocalories per one hundred grams, the ration is about 325 grams a day, roughly four 85 gram pouches.

3. 3

   Example three: a mixed diet for the same cat

   To cover half the requirement with wet food you need 130 kilocalories from wet, about 162 grams, and 130 kilocalories from dry. With a kibble at 3900 kilocalories per kilogram that is about 33 grams of dry food. Both portions need weighing, not eyeballing.

4. 4

   Cross check against the manufacturer table

   If your figure differs widely from the table, check the factor you used and the energy density. Coming out lower is normal for neutered, sedentary animals, since tables are built on averages that include entire and active ones.

Weight alone does not tell you whether an animal is in shape, because frame size differs. The 9 point body condition score solves this by assessing fat cover over ribs, spine and abdomen instead of an absolute number. The target is to stay between 4 and 5.

Body condition also lets you estimate ideal weight when it is unknown: starting from current weight and score, each point above 5 corresponds roughly to a 10 per cent excess. A 28 kg dog scoring 7 therefore has an ideal weight around 22 or 23 kg, and the weight loss calculation runs on that figure rather than the current 28.

The rate of loss needs watching. In dogs, one to two per cent of body weight per week is considered appropriate. In cats the pace must be slower, between 0.5 and one per cent per week, because rapid weight loss, and fasting even more so, risks serious liver disease. A cat that stops eating during a diet needs reassessing immediately.

The most precise calculation in the world falls apart if half the calories come from uncounted sources. The working rule is simple: treats, training rewards, palatable supplements and leftovers should not exceed ten per cent of daily kilocalories, and that ten per cent comes out of the ration rather than on top of it.

• On 1060 daily kilocalories for a 20 kg dog, the extras budget is about 106 kilocalories: far less than it seems once you read the labels on commercial biscuits.
• On 260 daily kilocalories for a neutered cat, the budget is about 26 kilocalories, meaning a few grams of dry treat.
• For training, take part of the daily ration and use it as rewards instead of adding a different food.
• Many dental chews and long lasting chews are energy dense and count as a full treat, not an accessory.
• In households with several people the biggest risk is double feeding: writing down who gave what solves a problem no calculation can fix.

When the ration is reduced for weight loss, meal volume drops and some animals start food seeking insistently. The answer is not to go back but to increase volume at the same calorie level: diet foods with lower energy density, added wet food, splitting into more meals a day. Satiety depends on volume and fibre content too, not only on kilocalories.

Every formula produces an estimate with individual error that can reach a fifth in either direction. That is why the calculation is a starting point rather than a destination: apply it, verify it against real weight, correct it. Without that loop even the most careful arithmetic can lead you astray.

1. 1

   Weigh before you start and record the date

   You need a zero point. Use the same scale each time, at the same hour and before a meal, because stomach contents and a full bladder move the number.

2. 2

   Run the calculated ration for two or three weeks

   Change nothing during that window unless there is a clinical reason. Frequent adjustments make it impossible to tell which variable produced the effect.

3. 3

   Reweigh and compare against the goal

   If weight is stable and body condition sits between 4 and 5, the ration is right. If it drifts in an unwanted direction, move to correction.

4. 4

   Correct by ten per cent, no more

   Raise or lower the ration by about a tenth and wait another two or three weeks. Abrupt corrections produce oscillation and, in cats, the risk of food refusal.

5. 5

   Redo the calculation at every change

   Neutering, the end of growth, entering the senior stage, starting or stopping sport, changing food and chronic disease all move the requirement and call for a fresh calculation.