Quick Answer
A commercial cooling tower typically uses 80,000 to 500,000 gallons of water per month during the cooling season, depending on system size, climate, and load. As a rule of thumb, a tower evaporates about 1.8 gallons per minute per 100 tons of cooling — so a 500-ton system at full load evaporates roughly 390,000 gallons per month, plus blowdown (about half the evaporation volume at 3 cycles of concentration) and a small amount of drift. Cooling towers commonly account for 20 to 40 percent of a commercial building’s total water use. Because 60 to 75 percent of tower water evaporates and never reaches the sewer, buildings without sewer credits are paying sewer charges on hundreds of thousands of gallons that go into the atmosphere.
If you’re responsible for managing a commercial building’s water costs, the cooling tower is almost certainly your single largest water consumer. But how much water does it actually use? The answer depends on your building’s size, climate, and how well your tower is maintained — and the numbers are often larger than people expect.
The Three Sources of Water Loss
A cooling tower loses water through three mechanisms, and understanding each one matters for both budgeting and sewer credit calculations. Evaporation is by far the largest — it’s the tower’s intended function, removing heat by evaporating water into the atmosphere. A typical commercial tower evaporates approximately 1.8 gallons per minute per 100 tons of cooling capacity. For a 500-ton system running at full load, that’s about 9 gallons per minute, or roughly 390,000 gallons per month.
Blowdown is the second-largest water loss. As water evaporates, the dissolved minerals left behind become increasingly concentrated. To prevent scale buildup, a portion of this concentrated water must be regularly drained and replaced with fresh water. Blowdown volume depends on your cycles of concentration — the ratio of dissolved solids in the circulating water versus the makeup water. At 3 cycles of concentration (a common target), blowdown volume is roughly half the evaporation volume. At 5 cycles, it drops to about a quarter.
Drift — the tiny water droplets carried out by the tower’s airflow — accounts for the smallest loss, typically 0.001 to 0.005 percent of the circulating water rate. While negligible compared to evaporation and blowdown, drift still contributes to total water consumption.
Real-World Consumption Numbers
According to the U.S. Department of Energy, cooling towers in commercial buildings typically consume 20 to 40 percent of the building’s total water supply. Here are some representative monthly consumption figures based on system size and climate:
A 200-ton system in a moderate climate (like Chicago or Philadelphia) typically uses 80,000 to 120,000 gallons per month during the cooling season. A 500-ton system in a hot climate (like Dallas or Phoenix) can use 300,000 to 500,000 gallons per month during peak summer. A 1,000-ton system serving a large office complex or hospital might consume 600,000 to over 1,000,000 gallons per month in warm climates.
These numbers include evaporation, blowdown, and drift combined. The cooling season in most U.S. markets runs five to eight months, though buildings with year-round cooling loads (data centers, hospitals, some manufacturing facilities) consume cooling tower water twelve months a year.
Why It Matters for Your Budget
At combined water and sewer rates that commonly range from $12 to $25 per thousand gallons in major U.S. cities, the monthly cost of cooling tower water is significant. A 500-ton system using 400,000 gallons per month at $18 per thousand costs $7,200 per month — $86,400 per year. And here’s the key insight: roughly 60 to 75 percent of that water is evaporation that never reaches the sewer system. Without sewer credits, you’re paying sewer charges on every gallon, including the ones that went into the atmosphere.
The EPA’s WaterSense program identifies cooling towers as one of the top three water consumers in commercial buildings, alongside restrooms and landscaping — and often the single largest.
How to Track Your Tower’s Actual Usage
The best way to know exactly how much water your cooling tower uses is with a dedicated submeter on the makeup water line. Without metering, you’re guessing based on industry averages and equipment specs. With metering, you have actual data — data that also serves as the foundation for sewer credit applications that can recover a significant portion of your cooling tower water costs.
Estimating Your Own Tower’s Monthly Usage
You can build a defensible estimate for your building in about ten minutes with three inputs: cooling capacity, average load, and cycles of concentration. Start with evaporation: multiply tonnage by 0.018 gallons per minute (the 1.8 gpm per 100 tons rule), then by your average load factor, then by minutes of monthly runtime. A 300-ton tower at 70 percent average load running 24/7 evaporates roughly 300 x 0.018 x 0.70 x 43,200 = about 163,000 gallons per month.
Then add blowdown using your cycles of concentration: blowdown = evaporation / (cycles – 1). At 3 cycles, that 163,000 gallons of evaporation carries about 82,000 gallons of blowdown; at 5 cycles, about 41,000. Total makeup is the sum of the two plus a small drift allowance. If your calculated estimate differs sharply from what a makeup submeter shows, that gap is diagnostic — it usually points to a stuck float valve, an overflowing basin, scale-driven inefficiency, or a blowdown valve that isn’t seating.
Seasonal Swings and What They Mean for Billing
Cooling tower water use is not flat across the year. In most U.S. climates, July and August consumption can run 3 to 5 times higher than April or October, because both runtime hours and heat load peak together. That seasonality has two practical consequences for anyone managing the budget.
First, don’t annualize a summer bill — or a winter one. Budget from a 12-month meter record, or at minimum apply monthly weighting based on cooling degree days for your region. Second, seasonality affects sewer credit economics: your credit is largest exactly when your bill is largest, so a building that starts its metering and application process in winter can have credits active before the expensive months arrive. Utilities also expect seasonal variation in your data — a submitted record showing flat month-to-month usage is a red flag that suggests estimated rather than measured readings.
Curious what continuous visibility looks like in practice? See how RPM’s 24/7 cooling tower monitoring works.
Ready to Find Out What You Could Save?
RPM Water Equity Solutions helps commercial facilities recover money lost to sewer billing assumptions. If your building has cooling towers, you may be paying sewer charges on water that never reaches the sewer system.
Request your free assessment today and find out how much you could recover.
Know Your Numbers
Your cooling tower’s monthly water consumption is almost certainly larger than you think, and the sewer charges on that water are almost certainly higher than they should be. The first step is measurement — submeter the makeup line and find out what you’re actually using. The second step is action — use that data to apply for sewer credits and stop paying sewer charges on water that evaporates into the sky.
