Cold plunge chiller sizing calculator
Use this calculator to determine the right chiller size (HP) for your cold plunge. Sizing correctly is the most important chiller decision — undersize and your chiller won't keep up; oversize and you've wasted money.
The sizing rule
Rule of thumb: 1/4 HP per 25 gallons of water for indoor plunges, 1/4 HP per 20 gallons for garage/outdoor plunges (higher ambient temperature).
Quick reference table
| Tub size | Indoor (70°F) | Garage (85°F) | Outdoor (95°F) |
|---|---|---|---|
| 25 gal | 1/4 HP | 1/3 HP | 1/2 HP |
| 50 gal | 1/2 HP | 1/2 HP | 3/4 HP |
| 75 gal | 3/4 HP | 3/4 HP | 1 HP |
| 100 gal | 3/4 HP | 1 HP | 1.5 HP |
| 150 gal | 1 HP | 1.5 HP | 2 HP |
| 200 gal | 1.5 HP | 2 HP | 2.5 HP |
Interactive calculator
Use the form below to calculate your required chiller size:
Chiller Size Calculator
Enter your values and click Calculate.
How the calculation works
The calculator uses three factors:
- Tub size (gallons): More water = more thermal mass = more HP required
- Ambient temperature: Higher ambient = larger temperature differential = more HP required
- Target water temperature: Lower target = larger delta T = more HP required
The formula:
- Base HP = (tub_size / 25) × 0.25 (based on 1/4 HP per 25 gal rule)
- Adjustment factor = 1 + max(0, (delta_T - 20) / 10 × 0.1) — adds 10% per 10°F above 20°F delta T
- Required HP = Base HP × Adjustment factor
- Recommended HP = rounded up to nearest standard size (1/4, 1/3, 1/2, 3/4, 1, 1.5, 2, 2.5, 3)
Standard chiller sizes
Chillers come in standard HP sizes. When in doubt, round up:
| HP | Typical tub size (indoor) | Typical tub size (garage) | Price range |
|---|---|---|---|
| 1/4 HP | 20-30 gal | 15-25 gal | $400-500 |
| 1/3 HP | 30-40 gal | 20-30 gal | $450-550 |
| 1/2 HP | 40-60 gal | 35-50 gal | $550-700 |
| 3/4 HP | 60-80 gal | 50-70 gal | $700-900 |
| 1 HP | 80-110 gal | 70-100 gal | $900-1,200 |
| 1.5 HP | 110-160 gal | 100-140 gal | $1,200-1,600 |
| 2 HP | 160-220 gal | 140-200 gal | $1,500-2,000 |
Our top chiller picks by size
- 1/3 HP chiller bundle — For 25-50 gal tubs
- EONIX 1/2 HP — Best overall for 50-gal builds
- 1/2 HP alternative — Backup option
- AS ColdPlunge 1 HP — For large 100+ gal tubs
Factors that affect chiller sizing
Beyond the basic calculation, consider:
- Insulation quality: Well-insulated tub needs less chiller HP
- Sun exposure: Direct sun adds 20-30% to cooling load
- Usage frequency: Daily use needs faster recovery (larger HP)
- Multiple users: Each user warms the water 2-3°F
- Cover quality: Good cover reduces heat loss by 50%
- Ambient humidity: High humidity reduces chiller efficiency
Common sizing mistakes
Mistake 1: Undersizing to save money
A 1/4 HP chiller on a 100-gal outdoor tub will run 18+ hours per day, may never reach target temp, and the compressor will die in 2-3 years from overwork. Saving $200 upfront costs you $600+ in early replacement.
Mistake 2: Not accounting for summer ambient
A chiller sized for 70°F ambient may not handle 95°F summer heat. Always size for your worst-case ambient temperature.
Mistake 3: Forgetting about multiple users
If 2+ people use the plunge daily, each session warms the water 2-3°F. Size up to handle the recovery load.
Mistake 4: Ignoring insulation
An uninsulated tub can need 2-3x the chiller HP of an insulated one. Always insulate before sizing chiller.
When in doubt, buy one size larger than the calculator suggests. The marginal cost is $100-200, and the upside (faster cooling, better heat wave handling, longer compressor life) is worth it. An oversized chiller also runs less often, which means less noise.
For the full chiller buyer's guide, see our chiller guide. For chiller troubleshooting, see our troubleshooting guide. For build instructions, see our master DIY build guide.