RD Exam Clinical Calculations: BMI, Weight Change, Mifflin-St Jeor, Protein, Fluids, Tube Feeding and PN

Where clinical math sits in the RD exam outline

CDR's study resources (the outline, a reference list, a study infographic and practice questions) include no formula sheet, and the 2026 handbook mentions only an on-screen calculator (a simple handheld one may be issued) and an erasable note board. The 2027–2031 test specifications show where clinical calculations come from, all in Domain III:

• Screening and assessment: anthropometric data and growth charts, intake and output, energy and nutrient requirements
• Planning and intervention: energy and nutrient needs specific to a condition, and, under enteral and parenteral nutrition, "formulas and calculations"

The worked examples below are original, written by Passy. Round only at the end, and round the way the question asks.

Unit conversions to get right first

A wrong conversion carries through every later step, so convert before you plug anything into a formula.

BMI and its categories

Formula: BMI = weight in kilograms ÷ (height in meters)².

Worked example. An adult is 5 feet 6 inches tall and weighs 154 pounds.

1. Height: 66 inches × 2.54 = 167.6 cm = 1.676 m
2. Weight: 154 ÷ 2.2 = 70 kg
3. BMI: 70 ÷ (1.676 × 1.676) = 70 ÷ 2.81 = about 24.9

CDC's adult categories (age 20 and older) put 24.9 in the healthy weight range. CDC also calls BMI a screening measure, to be considered with other factors:

For ages 2 through 19, CDC uses BMI-for-age percentiles: overweight is the 85th to less than the 95th percentile, and obesity is the 95th percentile or greater. CDR's own practice set applies exactly this to an adolescent at the 90th percentile.

Weight change and percent of usual body weight

Formulas:

• Percent weight change = (usual weight − current weight) ÷ usual weight × 100
• Percent of usual body weight = current weight ÷ usual weight × 100

Worked example. Usual weight 160 pounds, current weight 148 pounds: (160 − 148) ÷ 160 × 100 = 7.5% loss, or 92.5% of usual body weight.

The time frame matters as much as the percentage. Malnutrition criteria such as GLIM judge weight loss within set time periods (the RD exam Domain III guide covers the frameworks), so note the dates in the stem.

Estimating energy needs with Mifflin-St Jeor

The equation comes from a 1990 study that measured resting energy expenditure (REE) by indirect calorimetry in 498 healthy adults, normal-weight and obese. The published simplified forms:

• Men: REE = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
• Women: REE = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161

Worked example. A 52-year-old man, 178 cm tall, weighs 90 kg.

1. 10 × 90 = 900
2. 6.25 × 178 = 1,112.5
3. 5 × 52 = 260
4. REE: 900 + 1,112.5 − 260 + 5 = 1,757.5, or about 1,758 kcal a day

The equation estimates resting needs. If a question gives an activity or stress factor, multiply by it: with a factor of 1.2, 1,757.5 × 1.2 = about 2,109 kcal. The 1.2 here is only an example. Use the factor the question gives, since published factors vary. Swapping the men's +5 for the women's −161, or the reverse, is an easy way to lose the point.

Other questions express energy needs as kcal per kilogram. Then the trap is which weight to use: CDR's own practice set includes a critical-care item where the answer depends on a guideline that bases needs on ideal rather than actual body weight. Read for the weight the stem or guideline specifies.

To check your setup against a worked answer, you'll be able to see how planned explanations work Mifflin-St Jeor problems step by step once full prep launches.

Protein needs in grams per kilogram

Formula: protein (g/day) = body weight (kg) × target (g/kg).

The Dietary Reference Intakes set the adult RDA for protein at 0.8 g per kilogram of reference body weight, for healthy adults. Illness changes the target: CDR's own practice set includes a hemodialysis case whose keyed answer is to increase protein intake. When a question gives a target, the arithmetic is simple: at 1.2 g/kg, a 70-kg adult needs 70 × 1.2 = 84 g a day.

Fluid estimates: two common methods

ASPEN's PN order-writing basics list several ways to estimate fluid needs, including these two. Both are estimating rules, not measurements:

• 1 mL per kcal. A 1,900-kcal goal gives about 1,900 mL a day.
• Holliday-Segar (weight-based). 100 mL/kg for the first 10 kg, then 1,000 mL plus 50 mL/kg for each kg between 10 and 20, then 1,500 mL plus 20 mL/kg for each kg above 20. For 70 kg: 1,500 + (50 × 20) = 2,500 mL a day.

Different methods give different answers, so use the one the question names. ASPEN's material also notes that some conditions call for less than maintenance fluid and others for more.

Tube feeding: volume, rate, protein and free water

Steps:

1. Volume = energy goal ÷ formula kcal per mL
2. Rate = volume ÷ hours of infusion (continuous or cyclic), or volume ÷ number of feedings (bolus)
3. Protein = volume in liters × grams of protein per liter
4. Free water = volume × the formula's water fraction, which the question or label provides
5. Extra water = fluid goal − free water from the formula, split into flushes

Worked example. Goal 1,500 kcal. The formula provides 1.5 kcal/mL and 64 g protein per liter and is 76% free water (values given for this example; in a real question, use the label values in the stem). Fluid goal 1,800 mL.

1. Volume: 1,500 ÷ 1.5 = 1,000 mL
2. Continuous over 24 hours: 1,000 ÷ 24 = 41.7, about 42 mL an hour. Cycled over 16 hours overnight: 62.5, about 63 mL an hour. As 5 bolus feedings: 200 mL each
3. Protein: 1.0 × 64 = 64 g
4. Free water: 1,000 × 0.76 = 760 mL
5. Extra water: 1,800 − 760 = 1,040 mL, for example 4 flushes of 260 mL

This simple setup counts only the formula's water. If the stem lists other fluid sources, include them the way the question asks.

Explanations that show each step make these problems faster to fix when you miss one, and full prep is planned to cover them: you'll be able to check the planned tube-feeding rate and dialysis protein explanations at launch.

Parenteral nutrition calories

ASPEN's PN basics give the energy values:

Worked example 1, calories from a bag. 1,000 mL of 20% dextrose, 1,000 mL of 10% amino acids, and 250 mL of 20% lipid emulsion.

1. Dextrose: 20 g per 100 mL × 1,000 mL = 200 g, × 3.4 = 680 kcal
2. Amino acids: 100 g × 4 = 400 kcal
3. Lipid: 250 mL × 2 = 500 kcal
4. Total: 680 + 400 + 500 = 1,580 kcal

Worked example 2, dextrose by difference, the method ASPEN's material describes. Goal 1,800 kcal, protein 90 g, and 250 mL of 20% lipid.

1. Protein: 90 × 4 = 360 kcal
2. Lipid: 250 × 2 = 500 kcal
3. Remaining: 1,800 − 360 − 500 = 940 kcal
4. Dextrose: 940 ÷ 3.4 = about 276 g

Dextrose uses 3.4, not 4. Reaching for 4 kcal per gram, the usual value for dietary carbohydrate, is an easy slip in PN questions.

A tube-feeding rate practice question written by Passy

This is an original question in the exam's style. It isn't a CDR item.

A patient's energy goal is 2,000 kcal a day from a 1.5 kcal/mL formula, infused continuously for 20 hours each day. What hourly rate meets the goal?

• A. 56 mL per hour
• B. 67 mL per hour
• C. 83 mL per hour
• D. 100 mL per hour

Keyed answer: B. The volume is 2,000 ÷ 1.5 = about 1,333 mL, and 1,333 ÷ 20 = about 67 mL an hour. Option A spreads the volume over 24 hours instead of 20. Option C divides kilocalories, not milliliters, by 24. Option D divides kilocalories by 20 and skips the formula's concentration.

How to practice RD exam clinical math

1. Convert units first, then write the formula, then plug in numbers.
2. Ask which weight. Actual, usual or ideal weight changes the answer, so look for the weight the stem or the guideline it cites specifies.
3. Sanity-check. A tube-feeding rate of 400 mL an hour or a BMI of 9 means a slip.
4. Practice with a basic calculator, like the on-screen one, and try the Pearson demo test at pearsonvue.com/CDR.
5. Check targets against current references. Condition-specific energy and protein targets come from guidelines, so learn them from the clinical references your program used or those on CDR's reference list.

The 20 free RD exam practice questions include calculation items with explanations. If clinical math needs several extra weeks, compare the planned 90- and 180-day options for a longer math review. Management and foodservice formulas are in the RD exam foodservice math guide.