When humid air contacts an air conditioner’s cold evaporator coils, the water vapor condenses into liquid. This condensate either drips out of the unit or is directed into a dedicated drain line. In arid regions, many homeowners harvest this water for irrigation or other low‑water‑use applications.

Three key factors determine how much condensate a unit produces:

• Relative humidity – The higher the moisture content of the incoming air, the more condensate is formed.
• Airflow – Units that move more air pass more moisture to the coils per unit time.
• Air density – Denser air contains more water vapor for the same volume, increasing condensate output.

Step 1 – Determine the moisture mass flow

Multiply the unit’s airflow rate (gallons per minute) by the air’s specific humidity (pounds of water per pound of dry air). For example, a 12 gal/min unit with a specific humidity of 0.0065 lb / lb yields:

12 × 0.0065 = 0.078 lb water/min

Step 2 – Convert dry‑air volume to a mass basis

First, calculate the volume that one pound of dry air occupies (cubic feet). Then multiply by the conversion factor 8.33 lb / ft³ to obtain the equivalent mass in pounds. Using 400 ft³ as a typical value:

400 × 8.33 = 3 332 lb dry‑air

Step 3 – Compute condensate flow rate

Divide the moisture mass flow (Step 1) by the dry‑air mass (Step 2). The result is the condensate flow rate in gallons per minute:

0.078 ÷ 3 332 = 2.34 × 10⁻⁵ gal/min

TL;DR

1. Find your unit’s airflow in the owner’s manual.
2. Look up the local average relative humidity (see average relative humidity).
3. Convert that relative humidity to specific humidity using an online calculator (e.g., specific‑humidity calculator).
4. Apply the three‑step formula above to estimate condensate output.

Source: ASHRAE Handbook – HVAC Systems and Equipment, 2023 edition.