Renal ultrasound is one of the most requested studies in any general sonography department — and the measurements are deceptively nuanced. A kidney that's "normal size" in a small patient may be abnormal in a large one. A cortex that looks thin might be acute swelling or chronic scarring. Context is everything.

This guide gives you the renal ultrasound normal values you need, along with the clinical context to interpret them correctly.

Normal Kidney Size (Adult)

Length

Kidney length is the most commonly reported measurement and the primary indicator of renal size.

Parameter	Normal Range
Adult kidney length	9 – 12 cm
Right kidney (typical)	Slightly smaller than left
Left kidney (typical)	Often 0.5–1 cm longer than right

Important: Size must be interpreted relative to patient body habitus. A 9 cm kidney in a small adult woman may be normal; the same measurement in a 6-foot male may indicate atrophy. Renal length correlates with height and BSA.

Size Discrepancy Between Kidneys

A > 2 cm difference in kidney length between sides is considered significant and warrants comment. Causes include:

Renovascular hypertension (smaller kidney = renal artery stenosis)
Chronic pyelonephritis with unilateral scarring
Obstructive nephropathy
Congenital hypoplasia

Cortical Thickness

The renal cortex is the functional tissue — where filtration happens. Cortical thinning indicates nephron loss.

Measurement	Normal Value
Cortical thickness	≥ 10 mm
Cortical echogenicity	Hypoechoic to isoechoic compared to liver/spleen

Cortical Echogenicity Interpretation

Echogenicity	Normal?	Possible Significance
Hypoechoic to liver	Normal	—
Isoechoic to liver	Borderline	Medical renal disease (early)
Hyperechoic to liver	Abnormal	CKD, medical renal disease, nephrocalcinosis

Note: The right kidney is typically compared to the liver, and the left kidney to the spleen. Document echogenicity in those terms.

Renal Sinus and Pelvis

Structure	Normal Finding
Renal sinus	Echogenic (fat + vessels)
Renal pelvis AP diameter	≤ 10 mm (adult, non-obstructed)
Calyceal dilation	Absent in normal kidney

Hydronephrosis Grading

When the renal pelvis is dilated, document using standard grading:

Grade	Description
Grade 1 (mild)	Mild separation of sinus echoes; pelvis ≤ 10 mm
Grade 2 (moderate)	Moderate pelvic dilation with calyceal blunting
Grade 3 (moderate-severe)	Large pelvis, significant calyceal dilation, cortex maintained
Grade 4 (severe)	Severe dilation with cortical thinning

SFU grading (Society for Fetal Urology) is the most widely used system. Some institutions use a simple mild/moderate/severe description — follow your department's standard.

Renal Artery Doppler: Normal Values

Renal artery Doppler is performed to evaluate for renovascular hypertension and renal artery stenosis (RAS).

Direct Doppler (Renal Artery Origin)

Parameter	Normal Value
Peak systolic velocity (PSV)	< 180 cm/s
Renal-to-aortic ratio (RAR)	< 3.5
Waveform	Low-resistance (continuous forward diastolic flow)

RAS threshold: PSV > 180–200 cm/s at the renal artery origin AND RAR > 3.5 is consistent with hemodynamically significant renal artery stenosis (≥ 60%).

Indirect Doppler (Segmental / Interlobar Arteries)

When direct renal artery visualization is poor, indirect parameters are used:

Parameter	Normal Value	Abnormal
Resistive Index (RI)	0.58 – 0.70	> 0.70
Acceleration Time (AT)	< 70 ms	> 100 ms
Parvus-tardus waveform	Absent	Suggests proximal stenosis

Resistive Index Formula

RI = (PSV − EDV) / PSV

The RI reflects downstream resistance. Elevated RI (> 0.70–0.80) can indicate:

Renal parenchymal disease (CKD, acute rejection in transplant)
Renal vein thrombosis
Hydronephrosis

SonoBuddy's Resistive Index Calculator computes RI instantly from PSV and EDV.

Renal Transplant: Key Differences

Transplant kidneys sit in the iliac fossa and are more accessible to ultrasound. Normal transplant values differ slightly:

Parameter	Normal Transplant Value
RI	< 0.70
Kidney length	Variable — compare to baseline
Cortical echogenicity	Similar to native kidney

Elevated RI in a transplant kidney raises concern for:

Acute rejection
Cyclosporine toxicity
Renal vein thrombosis
Hydronephrosis

Other Structures to Evaluate

Structure	What to Assess
Urinary bladder	Pre- and post-void volumes (if requested)
Ureter	Normal = not visible; dilation suggests obstruction
Renal vessels	Patent, appropriate flow direction
Perinephric space	Fluid = hematoma, urinoma, lymphocele (in transplant)

Key Images for Your Worksheet

Right kidney — longitudinal with length measurement
Right kidney — transverse with width and AP measurements
Right kidney cortex — echogenicity compared to liver
Left kidney — longitudinal with length measurement
Left kidney — transverse with width and AP measurements
Left kidney cortex — echogenicity compared to spleen
Any hydronephrosis — grade and pelvis measurement
Renal Doppler — RI at interlobar arteries (bilateral)

Quick Reference Summary

Parameter	Normal Value
Kidney length	9 – 12 cm
Cortical thickness	≥ 10 mm
Cortical echogenicity	Hypoechoic to liver (right) / spleen (left)
Renal pelvis AP	≤ 10 mm
Renal artery PSV	< 180 cm/s
Renal artery RAR	< 3.5
Resistive Index	0.58 – 0.70
Acceleration Time	< 70 ms

Access Full Renal References in SonoBuddy

Open SonoBuddy → Measurements → Kidneys for the complete renal reference table. The Resistive Index calculator is under Calculators → Resistive Index.

References: SRU Consensus Guidelines. Tublin ME et al. Radiology (renal Doppler). AIUM Practice Parameters for Renal Ultrasound.

SonoBuddy is a reference tool, not a diagnostic authority. Clinical decisions must involve the ordering provider and interpreting physician.

Renal Ultrasound Normal Values: Kidney Size, Cortical Thickness, and More