Theoretical ice requirement
103 lb
About 11 × 10-lb bags.
Approximate bag cost: $38.50.
Assumes ice begins near 32°F and remains in the tub. Sun, air, plumbing, warm walls, drainage and imperfect mixing increase real-world requirements.
The heat-balance formula
Water weighs about 8.34 lb per gallon and requires roughly 1 BTU to cool 1 lb by 1°F. Ice near 32°F absorbs about 144 BTU/lb while melting, then the meltwater warms to the target temperature.
ice lb = [gallons × 8.34 × (start°F − target°F)] ÷ [144 + (target°F − 32)]
100-gallon example: 100 × 8.34 × (70 − 50) ÷ (144 + 18) = approximately 103 lb of ice. With 10-lb bags, round up to 11 bags.
Static reference table
Pounds of ice by tub size and temperature
Theoretical pounds, rounded to the nearest pound. Real-world use is normally higher.
| Gallons | Start | Target 50°F | Target 45°F | Target 39°F |
|---|---|---|---|---|
| 50 | 60°F | 26 lb | 40 lb | 58 lb |
| 50 | 70°F | 51 lb | 66 lb | 86 lb |
| 50 | 80°F | 77 lb | 93 lb | 113 lb |
| 50 | 85°F | 90 lb | 106 lb | 127 lb |
| 75 | 60°F | 39 lb | 60 lb | 87 lb |
| 75 | 70°F | 77 lb | 100 lb | 128 lb |
| 75 | 80°F | 116 lb | 139 lb | 170 lb |
| 75 | 85°F | 135 lb | 159 lb | 191 lb |
| 100 | 60°F | 51 lb | 80 lb | 116 lb |
| 100 | 70°F | 103 lb | 133 lb | 171 lb |
| 100 | 80°F | 154 lb | 186 lb | 226 lb |
| 100 | 85°F | 180 lb | 212 lb | 254 lb |
| 150 | 60°F | 77 lb | 120 lb | 174 lb |
| 150 | 70°F | 154 lb | 199 lb | 257 lb |
| 150 | 80°F | 232 lb | 279 lb | 340 lb |
| 150 | 85°F | 270 lb | 319 lb | 381 lb |