Catch-Can Uniformity Calculator

Turns a field catch-can (or rain gauge) test into the three uniformity coefficients irrigation practitioners actually use to judge a sprinkler or center-pivot system: Christiansen's CU, low-quarter Distribution Uniformity (DU), and the distance-weighted Heermann-Hein CU_H used for center pivots. Paste in your can readings, or enter them by hand.

1. Enter Catch-Can Data

Each can needs a catch amount. If your test is on a center pivot (cans placed along a radial line), also enter each can's distance from the pivot point — this is what CU_H needs to properly weight the area each can represents. For a lateral, solid-set, or hand-move test where every can represents equal area, leave distance blank (it will default to 1 for every can, which makes CU_H equal to CU).

Cans measured in volume (mL), not depth — convert using catch-can throat area

Catch-can throat (opening) area

in²

A standard 6-in diameter catch can has a throat area of π(3 in)² = 28.27 in².

Paste data (one can per line): distance, catch or just catch

Can #	Distance from pivot (optional)	Catch (in)

Set number of cans:

2. Results

About this calculator

DU (low-quarter distribution uniformity) compares the average of the driest 25% of the field to the field average — it answers "how bad is it where it's worst?"

DU = d_LQ / d_z × 100%

CU (Christiansen's uniformity coefficient) is based on the average deviation of every can from the mean — a more holistic, less worst-case view.

CU = 100% × (1 − &Sigma|d_i − d_z| / (n × d_z))

CU_H (Heermann-Hein, for center pivots) weights each can's deviation by its distance from the pivot, since a can far out represents far more irrigated area (a bigger annulus) than one close in.

CU_H = 100% × (1 − &Sigma S_i|d_i − d_z^*| / &Sigma S_id_i) where d_z^* = &Sigma d_iS_i / &Sigma S_i

Rules of thumb from the textbook: for moved-lateral sprinklers, CU ≥ 80 (or DU ≥ 0.70) is the usual minimum acceptable uniformity; for center pivots, CU_H ≥ 90 is often the target; for microirrigation, DU ≥ 0.80 is recommended.

Textbook Example 5.1: 20 catch cans (no distance weighting, since every can represents equal area) give DU = 0.75 and CU = 84%. Click "Load textbook Example 5.1" above to see it worked through.