Kidney STONE Calculator for Percutaneous Neprolithotomy

How to use this kidney STONE calculator

What this calculator does

This calculator converts five non-contrast CT findings into the S.T.O.N.E. nephrolithometry score used to describe kidney-stone complexity before percutaneous nephrolithotomy (PCNL). It then maps the total score to a published estimate of being stone-free after the first procedure: 94% for score 5, 92% for 6, 88% for 7, 83% for 8, 64% for 9, 42% for 10, and 27% for scores of 11 or more. The score is a preoperative stratification aid, not an individual prognosis, treatment recommendation, or substitute for a urologist's interpretation of the CT and clinical situation. The original five-feature system and its clinical intent are described in the original S.T.O.N.E. nephrolithometry study.

When to use it

Use the calculator when reviewing a CT before a planned PCNL, when explaining why two kidney-stone cases may differ in technical complexity, when documenting a standardized imaging-based score in a clinical or research workflow, or when checking how a changed CT feature would move the published score category. It is not designed to decide whether PCNL is appropriate, predict complications on its own, estimate recurrence risk, or replace local definitions of a stone-free outcome.

How to calculate

The calculator opens with a complete demonstration set – 300 mm² stone size, 80 mm tract length, no or mild hydronephrosis, 1 – 2 involved calyces, and 800 HU stone density – so the first result and an example XLSX workbook are immediately available.

1. Replace each demonstration value with the measurements or categories from the CT report. Numeric entries use a decimal point, not a decimal comma, and do not accept scientific notation or unit text inside the field.
2. Review the live STONE score, Estimated stone-free outcome after first PCNL, and Complexity band. The Score breakdown table shows exactly how each current input contributes points.
3. Select Download Excel to generate a fresh workbook from the current validated values. The workbook contains Summary, Inputs, Breakdown, and Score Guide sheets.
4. Select Reset to clear the demonstration numeric data and calculated content. Reset intentionally does not restore the sample. Download Excel is disabled until a complete valid set of measurements is entered again.

Input guide

Stone size is required and is entered in square millimetres (mm²). It is the stone's measured length multiplied by width. Enter a positive plain decimal such as 300. Values below 400 score 1 point; 400 – 799 score 2; 800 – 1599 score 3; and 1600 or more score 4. Increasing stone area can therefore raise the total in steps. A common mistake is entering a diameter in millimetres instead of area in square millimetres.

Tract length is required and is entered in millimetres. It represents the skin-to-stone distance used by the scoring system; 80 mm is a realistic example. A value at or below 100 mm scores 1 point and a value above 100 mm scores 2. Do not enter centimetres unless you convert them first; for example, 10 cm equals 100 mm.

Obstruction (hydronephrosis) is required and is categorical. Choose No or mild hydronephrosis for 1 point or Moderate or severe hydronephrosis for 2 points. This is an imaging category rather than a percentage. The main interpretation error is choosing a category from symptoms rather than the CT assessment.

Number of calyces with calculi is required and uses three options: 1 – 2 calyces for 1 point, 3 calyces for 2 points, or Full staghorn stone for 3 points. Broader collecting-system involvement increases the score. A full staghorn stone is a specific anatomic pattern; it should not be selected merely because a stone is large.

Stone density is required and is entered in Hounsfield units (HU), for example 800 HU. Density at or below 950 HU scores 1 point and density above 950 HU scores 2. Enter the CT attenuation value rather than a laboratory measurement or a guessed stone composition.

Output guide

STONE score is the exact sum of the five component points and ranges from 5 to 13 with this implementation of the published criteria. A lower total represents less imaging-defined complexity; a higher total represents more. Estimated stone-free outcome after first PCNL is a categorical percentage associated with the total score, not a personalized probability generated from individual patient covariates. Complexity band groups score 5 as Low, scores 6 – 8 as Moderate, and scores 9 – 13 as High to make the published outcome pattern easier to scan.

The Score breakdown table has four columns. Component names the S, T, O, N, or E feature in plain language. Current value shows the validated measurement or selected category. Points shows the contribution added to the total. Scoring rule states the threshold or category that produced those points. Zero is not a valid total because every component contributes at least one point; an incomplete or invalid input suppresses the calculated result rather than silently assigning zero.

Worked example

With the startup values, Stone size 300 mm² contributes 1 point, Tract length 80 mm contributes 1, No or mild hydronephrosis contributes 1, 1 – 2 calyces contributes 1, and Stone density 800 HU contributes 1. The calculation is therefore 1 + 1 + 1 + 1 + 1 = a STONE score of 5. The score-to-outcome table maps 5 to an estimated stone-free outcome of 94%, so the displayed Complexity band is Low. Those same typed values and results are written to the first-open XLSX workbook.

Learn more

The score was developed for PCNL, a procedure in which instruments reach the kidney through a small tract in the back. For procedure context, the U.S. National Institute of Diabetes and Digestive and Kidney Diseases provides a clear description of percutaneous nephrolithotomy and other kidney-stone treatments. The score also has published external assessment; see the single-procedure validation study for a stricter stone-free outcome definition.