The Schofield Equation Explained: Estimating Energy and Protein Needs

If you have ever typed your age, weight and activity level into an energy calculator and wondered where the answer came from, there is a good chance the Schofield equation was behind it. It is one of the most widely used ways to estimate how much energy an adult’s body uses at rest, and it is the starting point many Australian dietitians use before adjusting for illness, recovery or activity. This guide explains what it calculates, walks through a worked example, shows how protein needs are estimated alongside it, and is honest about how accurate any equation can be for one person.

What the Schofield equation actually calculates

Your body uses energy even when you are lying still: to keep your heart beating, to breathe, to maintain body temperature and to run your organs. That resting energy use is your basal metabolic rate. The Schofield equations, published by W. N. Schofield in 1985, predict it from just three things: your sex, your age group and your body weight. For adults, FAO’s 2001 energy report kept the weight-based Schofield equations as its standard.

BMR is not the same as the amount you need to eat. Movement, work and exercise add to it, and so does recovering from illness or injury. The usual way to get from BMR to a daily estimate is to multiply it by a physical activity level.

These are the weight-only equations as listed in the FAO/WHO/UNU 2001 report. Energy is usually reported in kilojoules in Australia, so the answer is multiplied by 4.184 (1 kcal = 4.184 kJ).

A worked example

Take a woman aged 45 who weighs 70 kg and is lightly active.

1. BMR: 8.126 × 70 + 845.6 = 1,414 kcal per day, which is about 5,918 kJ.
2. Activity: a lightly active person is often given a PAL around 1.4. 1,414 × 1.4 = about 1,980 kcal per day, or about 8,285 kJ.

That figure is an estimate of what keeps her weight steady, not a target to eat to. Two women of the same age and weight can differ by several hundred kilojoules a day, because muscle mass, genetics, hormones, medicines, sleep and illness all affect energy use and none of them is in the equation.

Choosing an activity level

The activity multiplier makes a big difference, which is why it is the part to think about most. Australia’s Nutrient Reference Values describe PAL as ranging from 1.2 for bed rest up to about 2.2 for very active people or heavy physical work. They note that a PAL below 1.4 is not compatible with moving around freely or earning a living, and that 1.75 and above is consistent with good health.

The four levels used in our calculator sit inside the range described above. The top level, 1.9, is a convenient round figure rather than a published category, so treat the whole thing as a guide.

How protein is estimated alongside it

Protein is usually worked out in grams per kilogram of body weight, not as a share of energy. Australia’s Nutrient Reference Values set the recommended dietary intake for adults aged 19 to 70 at 0.84 g per kg for men and 0.75 g per kg for women. For people over 70 the figures rise to 1.07 g per kg for men and 0.94 g per kg for women. Needs can be higher in illness, recovery and older age. ESPEN guidance for older people suggests at least 1 g per kg per day, and for hospital patients at least 1.2 g per kg per day. These are clinical guidelines for particular groups, not targets for healthy adults.

How accurate is it?

Better than guessing, but not precise for any one person. Three things are worth knowing.

• It was built for groups. Schofield’s own paper says the error of prediction at an individual level “must be high”, and FAO states that its energy values are for population groups rather than individuals.
• The original data had gaps. FAO notes that almost half of the data behind the adult equations came from studies on Italian men in the 1930s and 1940s, and questions have been raised about how universally the equations apply. A 2005 re-analysis (the Oxford equations by Henry) found the FAO/WHO/UNU equations overestimate BMR in many communities.
• Individual error can be large. In a study of 196 overweight and obese adults, the Schofield average matched measured resting energy well (8.30 MJ estimated against 8.09 MJ measured), but for a person with a BMI of 35 the estimate could sit anywhere from about 2.8 MJ above to 2.6 MJ below their measured value. In a small group of people with severe obesity, Schofield overestimated by about 7%. In an Australian study of people with traumatic injury, measured needs fell within 10% of the Schofield estimate in only 44% of cases.

That is why dietitians treat the number as a starting point. They check it against what is actually happening: how your weight, appetite, energy, strength and blood results respond over weeks, and what your medical team is monitoring. A 2003 survey of Australian dietitians found 67% mainly used the Schofield equations, and that the injury or stress factors applied varied a good deal from one clinician to the next.

When not to rely on a calculator

• Under 18, or pregnant or breastfeeding. Needs follow different rules.
• Serious illness, burns, major surgery or tube feeding, where needs change from day to day and are set by the treating team.
• Eating disorders or a history of them (see the note above).
• Weight loss. Schofield tells you roughly what maintains your weight. It does not tell you how much to cut, and cutting hard can cost you muscle, energy and mood.