The normal size of kidneys in children follows a growth curve and is closely related to the age and size of the child. Ethnic differences have also been shown, which may be an important consideration when interpreting measurements against a reference range 4.
In infants median (normal range between the 2.5th and the 97.5th percentile in brackets) renal lengths on ultrasound are as follows 5:
Neonates (<1 month old): 50.6 mm (normal range: 42.1-60.1 mm)
1 month old: 52.4 mm (normal range: 43.6-62.0 mm)
2 months old: 54.2 mm (normal range: 45.2-63.8 mm)
3 months old: 56.1 mm (normal range: 46.9-65.7 mm)
4 months old: 57.3 mm (normal range: 48.0-66.8 mm)
5 months old: 58.6 mm (normal range: 49.2-68.0 mm)
6 months old: 59.7 mm (normal range: 50.2-69.0 mm)
7 months old: 60.8 mm (normal range: 51.3-70.0 mm)
8 months old: 61.8 mm (normal range: 52.2-70.9 mm)
9 months old: 62.7 mm (normal range: 53.1-71.6 mm)
10 months old: 63.5mm (normal range: 53.9-72.2 mm)
11 months old: 64.2 mm (normal range: 54.6-72.8 mm)
In older children median (normal range between the 2.5th and the 97.5th percentile in brackets) sonographic renal lengths based on their age and sex are as follows 5:
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1 year old
male: 65.2 mm (normal range: 54.6-76.8 mm)
female: 64.3 mm (normal range: 51.6-77.8 mm)
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2 years old
male: 68.7 mm (normal range: 57.7-80.9 mm
female: 68.4 mm (normal range: 55.4-82.4 mm)
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3 years old
male: 72.5 mm (normal range: 60.9-85.4 mm)
female: 71.7 mm (normal range: 58.4-86.2 mm)
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4 years old
male: 76.5 mm (normal range: 64.5-90 mm)
female: 75.8 mm (normal range: 62.3-90.8 mm)
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5 years old
male: 79.7 mm (normal range: 67.4-93.7 mm)
female: 78.9 mm (normal range: 65.3-94.1 mm)
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6 years old
male: 83 mm (normal range: 70.4-97.4 mm)
female: 81.6 mm (normal range: 67.9-97 mm)
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7 years old
male: 85.9 mm (normal range: 73-100.7 mm)
female: 84.9 mm (normal range: 71.2-100.5 mm)
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8 years old
male: 89.2 mm (normal range: 76-104.4 mm)
female: 87.8 mm (normal range: 74.1-103.5 mm)
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9 years old
male: 91.3 mm (normal range: 78-106.7 mm)
female: 90.8 mm (normal range: 77.1-106.7 mm)
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10 years old
male: 94.2 mm (normal range: 80.5-110.2 mm)
female: 93.1 mm (normal range: 79.4-109.1 mm)
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11 years old
male: 96.9 mm (normal range: 83.1-113.2 mm)
female: 95.9 mm (normal range: 80.9-113.8 mm)
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12 years old
male: 99.9 mm (normal range: 85.9-116.6 mm)
female: 97.9 mm (normal range: 84.5-113.8 mm)
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13 years old
male: 102.5 mm (normal range: 88.3-119.4 mm)
female: 100.1 mm (normal range: 86.8-116 mm)
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14 years old
male: 104.8 mm (normal range: 90.5-121.9 mm)
female: 101.8 mm (normal range: 88.6-117.6 mm)
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15 years old
male: 107.8 mm (normal range: 93.2-125.5 mm)
female: 103.5 mm (normal range: 90.5-119.1 mm)
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16 years old
male: 110.3 mm (normal range: 95.6-128.3 mm)
female: 105.2 mm (normal range: 92.4-120.7 mm)
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17 years old
male: 112.8 mm (normal range: 98-131 mm
female: 107 mm (normal range: 94.6-122.1 mm)
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18 years old
male: 115.6 mm (normal range: 100.6-134 mm)
female: 108.3 mm (normal range: 95.9-123.4 mm)
The best predictor of renal length in children is height, regardless of sex 3. Limits of normal kidney length (millimeters) in children can be calculated using the following formulas 3:
2.5th percentile: 0.4 × height [cm] + 20.4
50th percentile: 0.5 x height [cm] + 28.2
97.5th percentile: 0.5 × height [cm] + 34.8
Note: the left kidney is usually slightly longer than the right.