Guide · 5 min read
Converting mL catch readings to inches of depth
Many professional catch devices are read by pouring the collected water into a graduated cylinder, which gives millilitres rather than a depth. Depth is what the uniformity and precipitation arithmetic needs, and the conversion is a single division: depth equals collected volume divided by the area of the device's opening, called the throat area.
The conversion is where real audits most often go wrong, because it mixes metric volume with imperial depth. This guide pins the arithmetic down.
The conversion
One cubic inch is 16.387 millilitres. So depth in inches equals millilitres divided by 16.387, then divided by the throat area in square inches. Worked check: 164 mL collected by a device with a 10 square inch throat is almost exactly 10 cubic inches over 10 square inches, which is 1 inch of depth.
For a circular throat, area is pi times the diameter squared divided by 4. A 4-inch diameter opening has a throat area of about 12.57 square inches, so 100 mL collected in that device is 100 / 16.387 = 6.10 cubic inches, and 6.10 / 12.57 = 0.49 inches of depth. Write the two-step division down once and reuse it for every can.
Measuring the throat area
Manufacturers of purpose-made catch devices publish the throat area, often printed on the device itself; use the published figure. For improvised containers, measure the inside diameter of the opening with a ruler or calipers at the rim where water actually enters, and compute the circular area.
Every device in one test must share the same throat area, or each reading needs its own conversion. Mixed containers with unrecorded areas cannot be salvaged after the fact.
Entering mL readings in Irrigaudit
Switch the catch unit to mL in the workspace and enter the throat area once for the zone; either type the area directly or enter a throat diameter and let the tool compute the circular area. Every reading is then converted to inches automatically before DU, CU, and precipitation rate are computed, and the CSV export carries both the raw mL readings and the converted depths.
Frequently asked questions
How do I convert millilitres in a catch can to inches of water depth?
Divide the millilitres by 16.387 to get cubic inches, then divide by the device's throat area in square inches. Example: 164 mL in a 10 square inch throat is about 1 inch of depth.
What is the throat area of a catch device?
The area of the opening that actually intercepts spray, measured at the rim. For a circular opening it is pi times the diameter squared divided by 4. Purpose-made devices publish it; for improvised containers, measure the inside rim diameter.
Can I mix different containers in one catch-can test?
Only if you record each container's throat area and convert each reading separately, which is error-prone. Identical devices with one shared throat area are strongly preferable.