Normal Adult Kidney Size
The normal adult kidney measures approximately 10–12 cm in length (pole-to-pole), with the left kidney typically 2–3 mm longer than the right kidney, and size varies significantly with body height, sex, age, and body mass index. 1, 2
Standard Measurements by Ultrasound and CT
- Kidney length (pole-to-pole): The median renal length is 11.2 cm on the left and 10.9 cm on the right in adults, with normal range approximately 10–12 cm 2
- Mean kidney length across populations is 10.4 ± 0.8 cm, with mean width 4.5 ± 0.6 cm and mean cortical thickness 1.6 ± 0.2 cm 3
- CT measurements show slightly different values: 108.5 ± 12.2 mm for the right kidney and 111.3 ± 12.6 mm for the left kidney (approximately 10.9 cm and 11.1 cm respectively) 1
- Parenchymal width is 15.4 ± 2.8 mm on the right and 15.9 ± 2.7 mm on the left, while cortical width is 6.6 ± 1.9 mm bilaterally 1
Side-to-Side Variation
- The left kidney is consistently larger than the right kidney by approximately 2–3 mm in length, with this difference being statistically significant 1, 2
- Right kidneys are smaller in width, cortical thickness, and overall volume compared to left kidneys 3
- Median renal volumes are 146 cm³ in the left kidney and 134 cm³ in the right kidney 2
Sex Differences
- Males have longer kidneys than females in absolute measurements, with the left kidney in males showing statistically significant greater length than the right 4
- In females, the left kidney is also longer than the right, but this difference does not reach statistical significance 4
- When adjusted for body size (relative renal length), sex differences disappear, indicating that apparent sex differences are primarily due to body size rather than intrinsic sex-related factors 4
- Renal size correlates significantly with sex and body mass index in multivariate analysis 3
Age-Related Changes
- In men, kidney length increases up to the fifth decade of life, then begins to decline 1
- From the seventh decade of life onward (age ≥60 years), there is a significant decrease in both absolute and relative renal length 4
- Renal size decreases with age almost entirely because of parenchymal reduction, not changes in the central echogenic area 2
- Both absolute and relative renal length show significant decline after age 59 years 4
Body Habitus Correlations
- Body height is the single best predictor of kidney length, with correlation coefficient of 0.29 5
- Body mass index (BMI) is one of the most significant independent predictors for kidney length, cortical width, and parenchymal width 1
- Renal volume correlates best with total body surface area (strongest correlation among all body measurements), while renal length correlates best with body height 2
- Body weight, body surface area, and total body water content all show correlation coefficients of 0.26–0.32 with kidney length 5
Relative Renal Length (Kidney-to-Body Height Ratio)
- Relative renal length (kidney length divided by body height) is a more precise measurement than absolute length because it eliminates sex and height differences until age 59 years 4
- Relative renal length does not show significant differences between sexes and does not manifest significant decrease with age until the seventh decade 4
- Body index is the most useful and simplest method of estimating kidney size as an adjunct to clinical decisions, given the strong correlation between kidney length and body height 5
Additional Influencing Factors
- Kidney position, number of renal arteries, renal artery stenosis, prior infarctions, parapelvic cysts, and absence of the contralateral kidney all significantly influence kidney dimensions 1
- Cortical width is influenced by age, kidney position, parapelvic cysts, number of renal arteries, and renal artery stenosis 1
- Parenchymal width is additionally influenced by rotation status of the kidney 1
Clinical Measurement Considerations
- Kidneys <9 cm in adults are definitively abnormal and suggest chronic kidney disease, though normal-sized kidneys do not exclude CKD 6
- Renal cortical thickness <7 mm indicates advanced CKD with poor prognosis, whereas thickness ≥7 mm suggests preserved renal parenchyma 7
- Measurement of renal length can be performed with the patient either prone or supine without significant difference in results 2
- Abdominal coronal CT section predicts kidney length more accurately than other radiological methods (IVP, ultrasound, or transverse CT), though all methods have prediction errors of 0.5–1.2 cm 5
Key Clinical Pitfall
- Assessment of kidney size should be made individually considering BMI, height, sex, age, and kidney position, rather than relying solely on population-based reference ranges, because these factors have complex and significant influences on kidney dimensions 1