Body mass index is the most used and most misused number in a care home nutrition record. It appears on admission, it drives the first step of the MUST score, and it is often calculated from a guessed height, a weight taken in a wheelchair, or a figure copied from a hospital letter. This guide explains how to calculate BMI properly for people who cannot stand, what the ranges mean for older and disabled adults, where BMI is misleading, and how to build a simple BMI risk assessment into the care plan.
The short answer
BMI is weight in kilograms divided by height in metres squared. In a care home the weight should come from calibrated scales, at the same time of day, in similar clothing, and the height from a measured value or a recognised estimate such as ulna length when the person cannot stand. For MUST, a BMI over 20 scores 0, 18.5 to 20 scores 1, and under 18.5 scores 2. BMI on its own does not show recent weight loss, muscle loss or fluid retention, so it must always be read alongside the weight trend and the person's clinical picture. Record it monthly with the weight, flag changes, and act on the trend rather than a single number.
How to calculate BMI
The formula is weight (kg) divided by height (m) squared.
A resident who weighs 58 kg and is 1.62 m tall has a BMI of 58 divided by 2.62 (1.62 multiplied by 1.62), which is 22.1.
Most care records systems, including Kiwi, calculate BMI automatically from the recorded weight and height. That removes arithmetic errors, but it does not fix a bad input. If the height is a guess or the weight is wrong, the BMI is wrong, however precise the decimal point makes it look.
Getting the weight right
- Use calibrated scales and record when they were last calibrated. Chair, hoist and platform scales all drift.
- Weigh at the same time of day, ideally in the morning before breakfast and after the person has used the toilet.
- Wear similar clothing each time and no shoes.
- Subtract equipment. If the person is weighed in a wheelchair or with a hoist sling, record the weight of the chair or sling and subtract it. Write down which chair or sling was used.
- Record the method with the weight: standing scales, chair scales, hoist or bed. A change of method can look like a change of weight.
- Recheck surprising results straight away rather than recording them and waiting a month.
Getting the height right when the person cannot stand
A height measured standing against a wall is best. Many residents cannot stand straight, and heights recorded on admission are often self-reported or copied from old records. BAPEN's MUST guidance sets out alternatives:
| Method | How | When to use |
|---|---|---|
| Measured height | Standing straight against a wall, without shoes, using a fixed measure | Whenever the person can stand straight |
| Recent documented height | A measured height from a recent clinical record | If reliable and recent |
| Self-reported height | The person tells you | Only if they are likely to know and it seems realistic |
| Ulna length | Measure the forearm from the elbow point to the wrist bone and use the MUST conversion table | When the person cannot stand, the most common care home method |
| Knee height or demi-span | Alternative measurements with conversion tables | If ulna length cannot be measured |
Measure the height once properly and record how it was obtained. It does not need repeating monthly unless there is a reason to think it has changed, for example after a spinal fracture.
When neither weight nor height can be measured
For some people, particularly those who are very frail, bedbound without hoist scales or have significant contractures, a reliable weight is not possible. MUST then allows mid-upper arm circumference to estimate the BMI category. A MUAC under 23.5 cm suggests a BMI below 20; over 32 cm suggests a BMI above 30. Record that you used MUAC and why, and use clinical judgement and other signs such as loose clothing, rings and dentures, and visible muscle loss.
What the BMI ranges mean
| BMI | Usual category | MUST BMI score |
|---|---|---|
| Under 18.5 | Underweight | 2 |
| 18.5 to 20 | Lower end of healthy, possible risk | 1 |
| Over 20 to 24.9 | Healthy weight | 0 |
| 25 to 29.9 | Overweight | 0 |
| 30 and over | Obese | 0 |
NICE advises lower thresholds for overweight and obesity for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, because health risks rise at a lower BMI. For these groups, a BMI of 23 to 27.4 is classed as overweight and 27.5 or more as obese.
Where BMI misleads in a care home
BMI is a screening number, not a diagnosis. In care home residents it is misleading in predictable ways:
- It hides recent weight loss. Someone who goes from a BMI of 28 to 24 in three months has lost a large amount of weight and is at risk, even though both numbers sit in a 'healthy' or 'overweight' band. That is why MUST scores weight loss separately.
- It hides muscle loss. Older people lose muscle and gain fat, so a normal BMI can sit alongside significant frailty.
- Fluid distorts it. Oedema in heart failure or kidney disease adds weight that is not nutrition.
- Amputation changes it. Weight and the BMI calculation both need adjusting after a limb amputation.
- Some conditions change body shape. In some people with a learning disability, scoliosis or contractures make height measurement difficult, and in some genetic syndromes typical body composition differs.
Read BMI with the weight trend, appetite, intake records and how the person looks and feels.
How BMI fits into MUST
BMI is step one of the Malnutrition Universal Screening Tool. Step two scores unplanned weight loss over the last three to six months, and step three scores acute illness with no intake for more than five days. The scores are added to give a risk level and an action plan. Our step-by-step guide to calculating a MUST score works through examples, and the nutrition and hydration guide covers what happens next.
Underweight and overweight both need a plan
Most care home nutrition work focuses on underweight and weight loss, and rightly so: malnutrition is common, often missed and slows recovery from everything. But obesity matters too, particularly in learning disability and mental health services, where some antipsychotic medicines cause weight gain and where weight-related conditions such as type 2 diabetes and sleep apnoea are common.
A plan for a person with a high BMI should be person-centred and positive: activity they enjoy, food they like in sensible portions, a review of medicines that may contribute, and attention to the person's own goals. It should never be imposed through blanket restrictions on food. Restricting access to food for someone who lacks capacity needs a best-interests decision and is a restriction to record. Our guide to restrictive practice explains why.
A BMI risk assessment template
A BMI risk assessment does not need to be long. Use the same structure as any other risk assessment, so it sits with the rest of the person's risk record:
| Section | What to record |
|---|---|
| Measurements | Weight, date, scales used, equipment subtracted; height and how it was obtained; BMI |
| Trend | Weights for the last six months and the percentage change |
| Hazard | Malnutrition, unintended weight loss, or weight gain and related health risks |
| Contributing factors | Appetite, swallowing, dentition, mood, medicines, pain, dementia, activity |
| Likelihood and severity | Scored on the home's usual matrix |
| Controls | Food-first measures, fortified meals, snacks, support at meals, activity, referrals |
| Monitoring | Weigh weekly or monthly, food chart if needed, MUST monthly |
| Referrals | GP, dietitian, speech and language therapy, dentist |
| Review date | And triggers for earlier review: weight change, illness, admission |
Our universal risk assessment template shows the scoring and version-control structure in full.
How often to weigh and record BMI
Monthly is the usual minimum for every resident, with MUST recalculated at the same time. Weigh weekly if the MUST score is 1 or more, after a hospital admission, after a significant illness, or when a new medicine that affects appetite or weight is started. BMI only needs recalculating when the weight changes, which in practice means every time the person is weighed. A system that calculates it automatically saves time and errors.
Worked example: Priya
Priya is 86, cannot stand, and is weighed monthly on chair scales. Her admission record shows a height of 1.60 m, self-reported. The new nurse measures her ulna at 23 cm, which the MUST conversion table gives as about 1.55 m for a woman over 65. With a weight of 52 kg, the BMI changes from 20.3 on the old height to 21.6 on the measured one.
That looks like better news, but the trend tells a different story. Priya weighed 57 kg four months ago: a loss of nearly 9 per cent. Her MUST weight-loss score is 1, so her overall MUST is 1 and she is at medium risk. The nurse updates her height record with the method, starts weekly weights and a food chart, and asks the GP to review her medicines. Without the trend, the improved BMI would have hidden a real problem.
Common mistakes
- Using a self-reported or copied height for someone who has clearly shrunk.
- Weighing in a wheelchair and forgetting to subtract it, or using a different chair each month.
- Treating a normal BMI as reassurance when the weight trend is falling.
- Recalculating BMI by hand and getting it wrong.
- Not recording the scales or method used.
- No plan for residents with a high BMI, or a plan based on blanket food restriction.
What good looks like on inspection day
The inspector picks two residents. For each, the record shows a height with the method used, monthly weights with the scales recorded, BMI and MUST calculated each month, a trend that someone has clearly looked at, and actions when it changed: weekly weights, a food chart, a dietitian referral, a GP review. Staff can say how a resident is weighed and what happens if the weight drops. If you want to see weights, BMI and MUST trended on a single profile, book a demo.
Final conclusion
BMI is only as good as the weight and height behind it. Measure height once properly, weigh consistently on calibrated scales and record the method, let the system do the arithmetic, and always read BMI with the weight trend. Do that, and BMI becomes what it should be: a quick, reliable first step that tells you who needs a closer look.
Frequently asked
How do you calculate BMI?
Divide weight in kilograms by height in metres squared. For example, 58 kg divided by (1.62 multiplied by 1.62) gives a BMI of 22.1.
How do you measure height for someone who cannot stand?
Use ulna length, measured from the point of the elbow to the prominent wrist bone, and convert it with the MUST conversion table. Knee height or demi-span are alternatives. Record which method you used.
What BMI is underweight for an older person?
Under 18.5 is underweight. For MUST, a BMI under 18.5 scores 2 and 18.5 to 20 scores 1, so a BMI of 20 or below already indicates some risk in a care home resident.
Is BMI enough to screen for malnutrition?
No. BMI does not show recent weight loss, muscle loss or fluid retention. That is why MUST also scores unplanned weight loss and acute illness, and why the weight trend matters more than a single BMI.
How often should care home residents be weighed?
At least monthly, and weekly if the MUST score is 1 or more, after a hospital admission or illness, or when a medicine that affects weight is started.
What if a resident cannot be weighed at all?
Use mid-upper arm circumference to estimate the BMI category, as MUST allows, and use clinical signs such as loose clothing and visible muscle loss. Record why weighing was not possible.
Sources
- BAPEN: The 'MUST' explanatory booklet and alternative measurements
- NICE guideline CG32 Nutrition support for adults
- NICE: BMI thresholds for people from Black, Asian and other minority ethnic groups (PH46 and NG246)
- NHS: BMI healthy weight guidance
