CKiD U25 eGFR Calculator

The CKiD U25 creatinine equation for children and young adults up to 25 years, with the age- and sex-specific k coefficient that the bedside Schwartz equation lacks — built to work across the paediatric-to-adult transition, where Schwartz and CKD-EPI disagree.

Last updated 16 Sep 2026

CKiD U25 creatinine eGFR

Age + sex + height + creatinine → eGFR

SexThe U25 coefficients are sex-specific from the start and diverge sharply through puberty: at 17 years the male k is about 20% higher than the female k, tracking the sex difference in muscle mass and therefore in creatinine generation.

Age (years)Age in years, decimals accepted. The equation is validated from 1 to 25 years. The k coefficient changes continuously with age below 18 and is a flat value from 18 to 25, so the answer moves with age even when nothing else changes.

Height (cm)Standing height, or length in a child too young to stand. Height is the surrogate for muscle mass that makes a creatinine interpretable in a growing child, and the equation is linear in it — a 5 cm error moves the answer by about 3%.

Creatinine unitsThe published equation takes creatinine in mg/dL. An SI value is divided by 88.4 first, which is the house factor this site uses for creatinine everywhere.

Serum creatinineIn µmol/L or mg/dL as selected. The equation was derived against IDMS-traceable enzymatic creatinine; a non-standardised or Jaffe assay with a positive bias will make the eGFR read low, and the effect is proportionally largest in a small child whose creatinine is genuinely low.

83mL/min/1.73 m²Example

A boy of 14 years, height 155 cm, serum creatinine 70 µmol/L

The CKiD U25 creatinine equation

eGFR = k × height (m) ÷ serum creatinine (mg/dL)

k, male: 39.0 × 1.008^(age − 12) for 1 to under 12 years; 39.0 × 1.045^(age − 12) for 12 to under 18; 50.8 from 18 to 25
k, female: 36.1 × 1.008^(age − 12) for 1 to under 12 years; 36.1 × 1.023^(age − 12) for 12 to under 18; 41.4 from 18 to 25

39.0 and 36.1: the male and female k at exactly 12 years of age, where both exponential terms equal 1. The function is continuous at 12 by construction — there is no step at the breakpoint, only a change of slope
1.008: the shared pre-pubertal slope. k rises by 0.8% per year of age in both sexes below 12, which is a gentle correction: the male k moves only from about 37.8 at 8 years to 39.0 at 12
1.045 and 1.023: the pubertal slopes, and where the sexes part company. A 4.5% annual rise in boys against 2.3% in girls tracks the divergence in muscle mass, and therefore in creatinine generation, through puberty. By 17 years the male k is about 48 and the female about 40
50.8 and 41.4: the flat adult k values from 18 to 25 years. They very nearly meet the pubertal curves at 18 — the male curve reaches 50.77 at just under 18 — so the transition is essentially smooth rather than a jump
height in metres: the published equation is defined with height in metres and creatinine in mg/dL. This page asks for centimetres and divides by 100, and converts an SI creatinine by dividing by 88.4, so that a British report can be entered as printed
bedside Schwartz, for comparison: eGFR = 0.413 × height (cm) ÷ creatinine (mg/dL), which is 41.3 × height (m) ÷ creatinine — a single constant for every child of any age and either sex. U25 replaces that one number with a function of age and sex, which is the whole of the difference
what it does not use: cystatin C. Pierce and colleagues published U25 cystatin C and combined equations alongside this one; this page implements the creatinine equation only, and a cystatin-based estimate is a separate calculation with its own coefficients

Worked example

A boy of 14 years, height 155 cm, serum creatinine 70 µmol/L
Creatinine in conventional units: 70 ÷ 88.4 = 0.792 mg/dL
Height in metres: 155 ÷ 100 = 1.55 m
He is 14, so k is on the male pubertal segment: k = 39.0 × 1.045^(14 − 12) = 39.0 × 1.0920 = 42.59
eGFR = 42.59 × 1.55 ÷ 0.792 = 83 mL/min/1.73 m²
The bedside Schwartz equation on the same numbers gives 0.413 × 155 ÷ 0.792 = 81 mL/min/1.73 m² — close here, because the male k at 14 happens to be near Schwartz's implicit 41.3
A girl of 14 with identical height and creatinine gets k = 36.1 × 1.023² = 37.78, and an eGFR of 74 — an 11% difference that Schwartz, having no sex term, cannot produce
The same boy at 17 years, grown to 175 cm with a creatinine of 85 µmol/L: k = 39.0 × 1.045⁵ = 48.60, eGFR = 48.60 × 1.75 ÷ 0.961 = 88. Schwartz would give 75 on those numbers, a 17% difference — and this is the age at which the two equations diverge most

The k coefficient across the age range

Computed from the published coefficients. Two features are worth noticing. The sexes are close and nearly flat before 12, then diverge sharply — by 16 the male k is 18% above the female. And both pubertal curves arrive at almost exactly the adult plateau at 18, so the equation hands over to its own adult segment without a step. The bedside Schwartz equation’s single implicit coefficient, 41.3, sits in the middle of this table and is right for nobody in particular.
Age (years)	Male k	Female k
2	36.0	33.3
5	36.9	34.1
8	37.8	35.0
11	38.7	35.8
12	39.0	36.1
14	42.6	37.8
16	46.5	39.5
17.9	50.6	41.3
18 to 25	50.8 (flat)	41.4 (flat)

U25 against the equations either side of it

The gap in this table is the problem U25 was built to close. A young person who moves from a paediatric to an adult service gets their eGFR recalculated by a different equation on the same creatinine, and the number changes without their kidneys changing. U25 spans the transition so that the same equation can be used either side of it.
Equation	Age range	What it uses	Where it struggles
Bedside Schwartz	Children	A single constant, 0.413, with height and creatinine	No age or sex term, so it cannot follow the pubertal rise in creatinine generation; diverges from U25 most in adolescent boys
CKiD U25 creatinine	1 to 25 years	Age- and sex-specific k, with height and creatinine	Derived in chronic kidney disease; an independent study found it imprecise where measured GFR is above 60 mL/min/1.73 m²
CKD-EPI 2021 creatinine	Adults 18 and over	Age, sex and creatinine — no height	Not validated in children; applying it below 18 is off-label, and it is discontinuous with paediatric estimates at the transition
Measured GFR	Any	An exogenous marker — iohexol, inulin, a radioisotope	Slow, costly and not always available, but it is the reference the estimates are judged against

Worked estimates from the engine’s own arithmetic

Rows two and one differ only in sex, and the estimates differ by 11% — a difference the bedside Schwartz equation cannot express at all. The last row is the range in which U25 was derived and performs best; the third row is the range in which an independent validation found it imprecise.
Child	Height	Creatinine	eGFR (mL/min/1.73 m²)
Boy, 14 years	155 cm	70 µmol/L	83
Girl, 14 years	155 cm	70 µmol/L	74
Boy, 8 years	128 cm	0.5 mg/dL	97
Boy, 17 years	175 cm	85 µmol/L	88
Man, 20 years	175 cm	1.2 mg/dL	74
Girl, 6 years	115 cm	120 µmol/L	29

One equation across the transition

Estimating glomerular filtration rate from creatinine means guessing how much creatinine the person generates, and in a child that guess is mostly about muscle. The bedside Schwartz equation solves it elegantly and cheaply: height stands in for muscle mass, and a single constant of 0.413 converts height over creatinine into an estimated GFR. It works well enough across much of childhood that it has been the paediatric standard for over a decade. What it cannot do is follow puberty. A fourteen-year-old boy and a fourteen-year-old girl of identical height generate creatinine at measurably different rates, and a seventeen-year-old boy differs from his eleven-year-old self by more than height alone accounts for. A single constant has to be wrong in both directions to be right on average.

The CKiD U25 equations replace that constant with a function. The form is unchanged — k times height in metres divided by creatinine in milligrams per decilitre — but k now depends on age and sex. Below twelve it rises gently, by 0.8% a year in both sexes. From twelve to eighteen the sexes separate: 4.5% a year in boys against 2.3% in girls, which is the divergence in muscle mass written as a coefficient. From eighteen to twenty-five it is flat, at 50.8 for men and 41.4 for women. The curves are continuous at twelve by construction, and they arrive at the adult plateau at eighteen close enough that there is effectively no step there either.

That last property is the point of the whole exercise, and it is worth stating plainly: the problem U25 was built to solve is a discontinuity, not an inaccuracy. A young person with chronic kidney disease moving from paediatric to adult nephrology has their eGFR computed by the bedside Schwartz equation on one side of the transition and by an adult CKD-EPI equation on the other. The two do not agree. The same creatinine, the same kidneys, the same week — and the reported eGFR steps up or down because the equation changed. That step can move a patient between KDIGO GFR categories, alter where they sit on a transplant waiting list, and undermine the one thing an eGFR series is genuinely good for, which is showing a trend. An equation validated from one year to twenty-five removes the step by spanning it.

Two limitations belong on the page with the coefficients. The first is the derivation population: U25 was developed in the Chronic Kidney Disease in Children cohort, so it is calibrated in children with chronic kidney disease and is an extrapolation elsewhere. An independent cross-sectional study of patients aged two to twenty with a measured GFR above 60 mL/min/1.73 m² found both U25 and CKD-EPI imprecise in that upper range, which matters because a reassuringly normal estimate is exactly where imprecision is least likely to be questioned. The second is the assay. The coefficients assume an IDMS-traceable enzymatic creatinine; a method with a positive bias makes the estimate read low, and the proportional effect is largest in the small child whose true creatinine is lowest. Neither limitation argues against using the equation. Both argue for reading a series rather than a point, and for remembering that a measured GFR exists when the decision is large enough to deserve one.

Frequently asked questions

What is the CKiD U25 equation?

A creatinine-based eGFR equation for children and young adults from 1 to 25 years: eGFR = k × height in metres ÷ creatinine in mg/dL, where k depends on age and sex. For males k is 39.0 × 1.008^(age−12) under 12 years, 39.0 × 1.045^(age−12) from 12 to under 18, and 50.8 from 18 to 25; for females 36.1 × 1.008^(age−12), 36.1 × 1.023^(age−12) and 41.4 respectively.

How does CKiD U25 differ from the bedside Schwartz equation?

Schwartz uses one constant, 0.413 with height in centimetres, for every child regardless of age or sex. U25 keeps the same form but makes the coefficient a function of age and sex, so it can follow the rise in creatinine generation through puberty and the difference between boys and girls. The two diverge most in adolescent boys.

Why was U25 developed to cover ages up to 25?

To remove a discontinuity. A young person transitioning from paediatric to adult nephrology had their eGFR recalculated by a different equation, and the reported value changed without any change in kidney function — enough, sometimes, to move them between KDIGO GFR categories. An equation validated from 1 to 25 years can be used consistently either side of the transition.

Does the U25 equation need height?

Yes. Height in metres is the surrogate for muscle mass that makes a creatinine interpretable in a growing person, and the equation is linear in it. This distinguishes U25 from the adult CKD-EPI equations, which use age and sex but not height.

What are the limitations of CKiD U25?

It was derived in a cohort of children with chronic kidney disease, so it is least well calibrated in healthy children; an independent study in patients aged 2 to 20 with measured GFR above 60 mL/min/1.73 m² found it imprecise in that range. It assumes an IDMS-traceable enzymatic creatinine, and like every creatinine-based estimate it lags the true GFR in acute kidney injury.

Is the U25 result the child’s actual GFR?

No. Like every eGFR it is indexed to 1.73 m² of body surface area, so it is not the child’s absolute clearance in mL/min. Where absolute clearance matters — dosing chemotherapy or an aminoglycoside, for instance — the estimate must be de-indexed to the child’s own body surface area.

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References

Pierce CB, Muñoz A, Ng DK, Warady BA, Furth SL, Schwartz GJ. Age- and sex-dependent clinical equations to estimate glomerular filtration rates in children and young adults with chronic kidney disease. Kidney Int. 2021;99(4):948–956 — the U25 equations and their k coefficients.
National Institute of Diabetes and Digestive and Kidney Diseases. eGFR Equations for Children, Adolescents and Young Adults. Bethesda: NIDDK — the CKiD U25 creatinine and cystatin C κ values by age and sex.
Schwartz GJ, Muñoz A, Schneider MF, et al. New equations to estimate GFR in children with CKD. J Am Soc Nephrol. 2009;20(3):629–637 — the bedside Schwartz equation and its 0.413 constant.
Björk J, Nyman U, Larsson A, Delanaye P, Pottel H. Estimation of the glomerular filtration rate in children and young adults by means of the CKD-EPI equation with age-adjusted creatinine values. Pediatr Nephrol. 2022;37(5):1073–1081.
Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int. 2024;105(4S):S117–S314 — GFR categories G1 to G5.

Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.