Quick Answer
Sewer averaging means your sewer charge is based on your winter water use instead of your current month’s water use. The logic: winter water mostly goes down the drain (indoor use), while summer water often goes on the lawn [1]. Utilities use a winter average to avoid charging you for outdoor watering that never reaches the sewer.
Why winter? Because outdoor watering is minimal in cold months, so your winter water use is a fair approximation of the wastewater going into the sewer system. Utilities set the sewer charge for the rest of the year based on a winter average of a few months.
Real example: Austin Water. Austin Water’s wastewater charge is based on your water use during the Wastewater Averaging period or your monthly water use, whichever is lower [1]. If your winter average is 6,000 gallons per month, the charge is $13.40 per 1,000 gallons (the tier-2 rate for 2,001+ gallons) plus the $11.26 monthly base charge: $91.66/month for sewer, for the entire year [1].
How Sewer Averaging Works
Sewer averaging decouples your sewer charge from your summer spikes. Rather than metering sewer flow directly (which is rare), utilities use your water meter reading as a proxy: most water that enters your home goes out through the drain. The key step is identifying which months represent indoor-only use.
“The amount of wastewater billed is based upon water usage during the Wastewater Averaging period, or monthly water consumption, whichever is lower,” according to Austin Water’s 2025–2026 residential rate sheet [1].
San Antonio Water System (SAWS) explains it plainly: “SAWS sewer charges are based on your average winter water use. So, the less water you use during the winter, the lower your monthly sewer charge will be for the next billing year” [2][3].
The Averaging Window
Different utilities use different windows. The key is that the window falls during the coldest months, when outdoor watering is minimal.
San Antonio Water System (SAWS): SAWS averages your water use over three consecutive billing periods between mid-November and mid-March. The exact schedule reads: “beginning after November 15 and ending on or about March 15 of each year” [3]. The annual window shifts slightly with billing cycles, and SAWS publishes the updated schedule each fall [2].
Austin Water: Austin’s rate sheet states that wastewater charges are based on the “Wastewater Averaging period,” but the specific months are not listed in the published rate document [1]. Check your Austin Water bill or call customer service (512-494-9400) to confirm the current averaging window months.
Other cities: Many municipalities use the same approach — a 90-day winter window (typically December–February or January–March) — but the exact months vary. Always check your local utility’s published tariff or rate sheet.
How the Average Is Applied
Once the utility calculates your winter average, it becomes the basis for your sewer charge each month until the next winter window resets it.
The calculation has two parts:
- A fixed base charge that covers the cost of maintaining the sewer system connection.
- A volume charge multiplied by your winter average.
Austin Water example:
- Winter average: 6,000 gallons per month
- Tier-2 volume rate: $13.40 per 1,000 gallons (2,001+ gallons)
- Volume portion: 6 × $13.40 = $80.40
- Monthly base wastewater charge: $11.26
- Total monthly sewer charge: $91.66 [1]
San Antonio Water System example:
- Winter average: 6,000 gallons per month
- Sewer Service Availability Charge: $10.00 (5/8" meter)
- Sewer Volume Charge: $2.539 per 1,000 gallons
- Volume portion: 6 × $2.539 = $15.23
- Total monthly sewer charge: $25.23 [3]
Austin’s rates are higher per gallon, but both achieve the same goal: the winter average locks in the sewer charge, so summer lawn watering does not inflate your sewer bill.
How to Lower Your Average
Since the winter average determines your sewer charge for the next year, fixing leaks during the winter window is the most effective way to reduce it.
Fix leaks. The average household wastes more than 9,300 gallons of water per year from leaks, and fixing easily corrected household leaks can save about 10 percent on your water bill, according to the EPA’s WaterSense program [4]. A running toilet during the winter averaging period can inflate your average for the entire next year.
Delay non-essential indoor projects. If you are planning a major indoor project that uses water (painting that requires flushing, deep cleaning, etc.), doing it outside the averaging window keeps your winter average lower.
Check for silent leaks. “An unrepaired leak could also cause your winter average to be higher,” SAWS warns [2]. Common culprits:
- Worn toilet flappers (most common, often silent)
- Dripping faucets
- Leaky showerheads
- Running irrigation systems left on during winter (SAWS notes that watering dormant grass “will not make it grow, no matter how much water you apply” [2])
Monitor your bill. If you notice an unusually high winter bill, check your water meter for continuous flow when no water is being used. A meter reading that changes over a two-hour dry period means you have a leak [4].
Cities That Do Not Average
Not every utility uses winter averaging. Some common alternatives:
- Annual averaging: The sewer charge is based on the average of the last 12 months of water use. This still excludes one-time spikes but does not provide the same seasonal benefit.
- Fixed allowance: A set number of gallons (e.g., 4,000 per month) is included in the base sewer charge, and usage above that is billed at the sewer rate.
- Flat rate: A single monthly sewer fee unrelated to water use, common in older rate structures.
- Direct sewer metering: Some newer multi-family or commercial buildings have separate sewer meters, allowing direct measurement instead of estimation.
Check your bill’s fine print or your utility’s tariff to see which method applies. The line labeled “Sewer,” “Wastewater,” or “Sanitary” usually includes a clue in its description.
Why is my sewer charge the same every month?
Does sewer averaging mean I am paying for summer watering?
How do I find my winter average?
What if I had a leak during the averaging period?
Is sewer averaging the same everywhere?
Related Reading
- How Sewer Charges Work on Your Water Bill — the charge itself, line by line
- Why Is My Water Bill Higher in Winter? — the winter averaging season’s bookends
- How to Lower Your Water Bill — usage moves the average too
- Water Bill Too High? — the full diagnostic
Sources
- Austin Water 2025–2026 Water & Wastewater Rates, Residential Wastewater Customers — wastewater averaging basis, tier rates ($6.30/$13.40 per 1,000 gal), monthly base charge ($11.26).
- San Antonio Water System — Winter Averaging — explains how winter averaging works, mid-November to mid-March window, tips to lower average.
- SAWS Residential Water & Sewer Rates — sewer availability charge ($10.00), volume charge ($2.539/1,000 gal), averaging window dates.
- U.S. Environmental Protection Agency WaterSense — Fix a Leak Week — household leaks waste 9,300+ gallons per year, fixable leaks save ~10% on water bills.
