By SCWS Team
Published February 1, 2025 · 18 min read
An undersized pressure tank is the #1 reason pumps die young. Every time your pump cycles on and off, it draws massive startup current and generates heat. A properly sized tank means fewer cycles, cooler motors, and pumps that last 15-20 years instead of 5-8.
This comprehensive guide walks you through the sizing calculations, factors to consider, and recommendations for every situation.
Quick Sizing Guide
Table of Contents
The 1-Minute Rule
Your pump should run for at least 1 minute each time it cycles on. If it runs for less, your tank is too small. Count the seconds from when the pump kicks on to when it shuts off—that simple test tells you everything.
Why Pressure Tank Size Matters
The pressure tank is one of the most critical components in your well system, yet it's frequently undersized or misunderstood. Choosing the wrong size can lead to serious consequences:
Problems with Undersized Tanks
- Pump Short-Cycling: The pump turns on and off too frequently, causing excessive wear
- Reduced Pump Life: Motors are designed for limited starts per hour; excessive cycling burns them out prematurely
- Pressure Fluctuations: Noticeable drops in pressure during showers or when multiple fixtures run
- Higher Energy Costs: Pump motors draw maximum current during startup; frequent starts waste electricity
- Water Hammer: Rapid pressure changes can cause banging pipes
The Pump Protection Factor
Submersible well pumps are expensive—$1,500 to $4,000+ for replacement including labor. The #1 cause of premature pump failure is short-cycling. A properly sized pressure tank ensures your pump runs long enough each cycle to stay cool and reduces total cycles, potentially adding 5-10 years to pump life.
Industry Rule of Thumb:
Your pump should run for a minimum of 1 minute each time it cycles on. Preferably 2-3 minutes. If your pump cycles more than 6 times per hour during normal use, your tank is likely undersized.
How Pressure Tanks Work
Understanding how pressure tanks function is essential for proper sizing. Modern tanks use a bladder or diaphragm to separate compressed air from water.
The Operating Cycle
- Empty State: When the tank is "empty" (at cut-in pressure), it's actually full of compressed air at the pre-charge pressure
- Pump On: When pressure drops to cut-in (e.g., 40 PSI), the pump turns on
- Filling: The pump forces water into the tank, compressing the air bladder
- Pump Off: When pressure reaches cut-out (e.g., 60 PSI), the pump shuts off
- Delivery: Compressed air pushes water out to fixtures without the pump running
- Drawdown: The usable water delivered between pump cycles is called "drawdown"
Key Pressure Settings
| Setting | 40/60 System | 30/50 System | Purpose |
|---|---|---|---|
| Air Pre-Charge | 38 PSI | 28 PSI | 2 PSI below cut-in |
| Cut-In Pressure | 40 PSI | 30 PSI | Pump turns ON |
| Cut-Out Pressure | 60 PSI | 50 PSI | Pump turns OFF |
| Differential | 20 PSI | 20 PSI | Pressure swing range |
Critical:
The air pre-charge must be set 2 PSI below the pressure switch cut-in setting. If the pre-charge is wrong, the tank won't deliver its rated drawdown, leading to short-cycling even with a properly sized tank.
The Sizing Formula
The fundamental sizing formula balances pump output against desired run time:
Step-by-Step Calculation
Example: Sizing for a 10 GPM Pump
-
Determine pump flow rate: 10 GPM
(Check pump nameplate or installation records)
- Set minimum run time: 1 minute (minimum), 2 minutes (recommended)
-
Calculate required drawdown:
10 GPM × 1 minute = 10 gallons minimum drawdown
10 GPM × 2 minutes = 20 gallons recommended drawdown -
Select tank size: Choose a tank rated for at least 20 gallons drawdown
An 80-gallon bladder tank provides approximately 20-25 gallons drawdown on a 40/60 system
Alternative Quick Formula
Many professionals use this simplified rule:
So for a 10 GPM pump: 10 × 4 = 40-gallon tank minimum. For better pump protection, go up one size (52-gallon or 80-gallon).
Understanding Drawdown
Drawdown is the most misunderstood pressure tank specification. The tank's nominal size (e.g., "40 gallons") is NOT the amount of water it delivers. Drawdown is the usable water delivered between pump cycles.
Why Drawdown Is Less Than Tank Size
A pressure tank contains both air and water. The air compresses as water enters. At any given moment, the tank contains:
- Compressed air (approximately 40-60% of volume)
- Water under pressure (approximately 40-60% of volume)
Drawdown is only the water that can be used before the pressure drops to the cut-in point—typically about 25-30% of the tank's total gallon rating.
Drawdown by Tank Size (40/60 PSI System)
| Tank Size | Typical Drawdown | Run Time @ 10 GPM | Best For |
|---|---|---|---|
| 14 gallon | 3-4 gallons | 18-24 seconds | Booster systems only |
| 20 gallon | 5-6 gallons | 30-36 seconds | 1-2 bedroom, low GPM |
| 32 gallon | 8-10 gallons | 48-60 seconds | Small home, 5-7 GPM pump |
| 40 gallon | 10-12 gallons | 60-72 seconds | Average home, 8-10 GPM |
| 52 gallon | 13-16 gallons | 78-96 seconds | 3-4 bedroom, 10-12 GPM |
| 80 gallon | 20-25 gallons | 2-2.5 minutes | Large home, 12-15 GPM |
| 120 gallon | 30-36 gallons | 3-3.5 minutes | Very large, irrigation |
Complete Sizing Chart
This chart shows recommended tank sizes based on pump flow rate and desired minimum run time:
Recommended Tank Size by Pump GPM
| Pump GPM | Minimum (1 min) | Recommended (1.5 min) | Optimal (2 min) |
|---|---|---|---|
| 5 GPM | 20 gallon | 32 gallon | 40 gallon |
| 7 GPM | 32 gallon | 40 gallon | 52 gallon |
| 10 GPM | 40 gallon | 52 gallon | 80 gallon |
| 12 GPM | 52 gallon | 80 gallon | 80 gallon |
| 15 GPM | 80 gallon | 80 gallon | 120 gallon |
| 20 GPM | 80 gallon | 120 gallon | 2 × 80 gallon |
| 25+ GPM | 120 gallon | 2 × 80 gallon | 2 × 120 gallon |
Household Size Factors
Beyond pump GPM, your household's water demand patterns should influence tank sizing:
Water Usage by Household Size
| Household | Peak Demand | Common Scenario | Suggested Tank |
|---|---|---|---|
| 1-2 People | 5-8 GPM | 1 shower + toilet | 32-40 gallon |
| 3-4 People | 8-12 GPM | 2 showers + appliance | 40-52 gallon |
| 5-6 People | 12-18 GPM | Multiple simultaneous uses | 80 gallon |
| 7+ People | 18-25 GPM | Heavy concurrent use | 80-120 gallon |
Additional Factors to Consider
Size UP if you have:
- • Multiple bathrooms (3+)
- • High-flow shower heads
- • Large soaking/jetted tubs
- • Irrigation system
- • Pool with auto-fill
- • Livestock watering
- • Home-based business using water
- • Plans to add bathrooms later
Standard sizing is fine if:
- • 1-2 bathrooms
- • Low-flow fixtures
- • Normal household use only
- • No irrigation or minimal landscape
- • Consistent, predictable usage patterns
- • Limited space for larger tank
Flow Rates for Common Fixtures
| Fixture | Flow Rate (GPM) |
|---|---|
| Shower (standard) | 2.0-2.5 |
| Shower (rain/high-flow) | 3.0-5.0 |
| Bathtub fill | 4.0-6.0 |
| Kitchen faucet | 1.5-2.2 |
| Bathroom faucet | 1.0-1.5 |
| Toilet (during fill) | 3.0-4.0 |
| Dishwasher | 1.5-2.0 |
| Washing machine | 2.0-3.5 |
| Garden hose (½") | 4.0-6.0 |
| Sprinkler zone | 5.0-15.0 |
Tank Types Compared
Bladder Tanks
The most common type for residential wells. A replaceable rubber bladder separates air and water.
Advantages
- ✓ Bladder can be replaced
- ✓ Air and water never mix
- ✓ Maintains pre-charge longer
- ✓ Available in many sizes
- ✓ Most common, widely available
Disadvantages
- ✗ Bladder can fail (10-15 years typical)
- ✗ Replacement bladders can be expensive
- ✗ Slightly lower drawdown than diaphragm
Diaphragm Tanks
A permanent rubber diaphragm divides the tank. Less common but very reliable.
Advantages
- ✓ Very durable, long-lasting
- ✓ Slightly higher drawdown
- ✓ No internal moving parts
- ✓ Often better warranties
Disadvantages
- ✗ Cannot replace diaphragm
- ✗ Must replace entire tank if fails
- ✗ Fewer size options
- ✗ Higher upfront cost
Galvanized (Plain Steel) Tanks
Older technology without a bladder. Air and water are in direct contact.
Advantages
- ✓ No bladder to fail
- ✓ Can last 20-30 years
- ✓ Lower cost
- ✓ Simple design
Disadvantages
- ✗ Air absorbs into water over time
- ✗ Requires air volume control
- ✗ More maintenance
- ✗ Less efficient
- ✗ Can rust internally
Our Recommendation:
For most residential applications, a quality bladder tank from a reputable manufacturer (Flexcon, Well-X-Trol, Flotec) offers the best balance of performance, serviceability, and cost. Choose stainless steel fittings if your water is corrosive.
Special Sizing Situations
Irrigation Systems
If your well serves both household and irrigation, size the tank based on the irrigation demand (typically the higher of the two). Consider:
- A separate, larger tank dedicated to irrigation
- Multiple tanks in parallel for high-demand systems
- A storage tank + booster pump setup for very large systems
Low-Yield Wells
If your well produces less than your peak demand (e.g., a 3 GPM well serving a family of 4), you need a different approach:
- Add a storage tank (300-1,500 gallons) that fills during low-use periods
- The pressure tank then feeds from the storage tank via a booster pump
- This allows the well pump to run at its sustainable rate while meeting peak demands
Variable Frequency Drive (VFD) Systems
If you have a constant-pressure (VFD) system, tank sizing requirements change:
- VFD systems need a small tank (20-40 gallon) as a buffer
- The VFD varies pump speed to maintain constant pressure
- Large tanks are counterproductive—the VFD handles demand changes
- Consult your VFD manufacturer for specific tank sizing
Multiple Tanks
When one tank isn't enough, you can install multiple tanks in parallel. Benefits include:
- Combined drawdown from all tanks
- Redundancy if one tank fails
- Easier to fit in tight spaces than one large tank
- Can add capacity incrementally
Important: Tanks in parallel must be at the same elevation and properly piped to equalize pressure.
Installation Considerations
Space Requirements
| Tank Size | Approximate Dimensions | Weight (Full) |
|---|---|---|
| 20 gallon | 16" × 24" | ~190 lbs |
| 40 gallon | 16" × 38" | ~360 lbs |
| 52 gallon | 16" × 48" | ~460 lbs |
| 80 gallon | 22" × 48" | ~700 lbs |
| 120 gallon | 24" × 60" | ~1,050 lbs |
Location Requirements
- Level surface: Floor must support full weight of tank
- Frost protection: Must be in heated space or below frost line
- Access: Need room for maintenance and eventual replacement
- Drainage: Floor drain nearby recommended for service/failure
- Ventilation: Good air circulation for longevity
Typical Installation Costs
| Tank Size | Tank Cost | Installation Labor | Total Installed |
|---|---|---|---|
| 20 gallon | $150-$250 | $200-$400 | $350-$650 |
| 40 gallon | $250-$400 | $250-$450 | $500-$850 |
| 52 gallon | $350-$500 | $300-$500 | $650-$1,000 |
| 80 gallon | $500-$800 | $350-$600 | $850-$1,400 |
| 120 gallon | $800-$1,200 | $400-$700 | $1,200-$1,900 |
Need Help Sizing Your Pressure Tank?
Our experienced technicians can evaluate your pump specifications, water usage patterns, and specific needs to recommend the optimal pressure tank size. We provide professional installation with proper pressure settings and testing.
Frequently Asked Questions
What size pressure tank do I need for my well?
The right pressure tank size depends on your pump's flow rate and desired run time. The formula is: Tank Size = Pump GPM × Run Time (minutes) × 4. For example, a 10 GPM pump with a 1-minute run time needs a 40-gallon tank. Most homes with 3-4 bedrooms do well with a 40-80 gallon tank.
What is drawdown and why does it matter?
Drawdown is the amount of usable water a pressure tank delivers before the pump kicks on. A 40-gallon tank typically has about 10-12 gallons of drawdown. Larger drawdown means fewer pump cycles, extending pump life. The rule of thumb: you want at least 1 minute of run time at your peak demand rate.
Is a bigger pressure tank always better?
A larger tank offers benefits like fewer pump cycles and better pressure stability, but there's a point of diminishing returns. An oversized tank costs more upfront and takes up more space without proportional benefits. Generally, matching tank size to pump output (1 gallon of drawdown per GPM) provides optimal balance.
What's the difference between bladder and diaphragm pressure tanks?
Bladder tanks have a replaceable rubber bladder that separates air and water, allowing easy maintenance if the bladder fails. Diaphragm tanks have a fixed diaphragm that cannot be replaced—the entire tank must be replaced if it fails. Bladder tanks are more common for residential use due to serviceability.
How many gallons of water can I draw from a 40-gallon pressure tank?
A 40-gallon bladder tank typically provides about 10-12 gallons of drawdown (usable water) on a 40/60 PSI pressure switch setting. The remainder is air charge space. An 80-gallon tank provides approximately 20-25 gallons of drawdown. Actual drawdown depends on the pressure switch differential.
Can a pressure tank be too big for my pump?
A tank cannot be too big in terms of damage, but an oversized tank is unnecessarily expensive and takes up extra space. The tank should be sized to your pump's output, not much larger. A reasonable guideline: tank drawdown in gallons should roughly equal pump flow in GPM for a 1-minute minimum run time.
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