Hydronic systems live or die in the loop math. The sizing tool below shows the relationships: driveway dimensions × tubing diameter × spacing = circuit count, loop length, and flow requirement. Every number feeds the pump and manifold selection.
| Driveway Width (ft) | Driveway Length (ft) | Tubing Diameter | Spacing | Target Circuit Length (ft) | Glycol Mixture % | Total Tubing Needed (ft) | Balanced Circuit Loops Required | Area Covered per Loop (sq. ft.) | Peak Heat Load per Loop (Btu/h) | Target Flow Rate (GPM) | System Head Loss (ft of water) | Source |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 20 | 50 | 3/4" standard | 9" on-center | 250 | 50% Propylene Glycol | 1,334 | 6 | 167 | 25,050 | 2.2 | 13.9 | 1 |
| 20 | 50 | 3/4" standard | 8" on-center | 250 | 50% Propylene Glycol | 1,500 | 6 | 167 | 25,000 | 2.2 | 13.9 | 2 |
| 20 | 50 | 3/4" standard | 12" on-center | 250 | 50% Propylene Glycol | 1,000 | 4 | 250 | 37,500 | 3.3 | 13.9 | 3 |
| 20 | 30 | 3/4" standard (Inferred) | 9" on-center (Inferred) | 250 | 50% Propylene Glycol (Inferred) | 800 (Inferred) | 4 (Inferred) | 150 (Inferred) | 22,500 (Inferred) | 2.2 (Inferred) | 13.9 (Inferred) | 4, 5 |
| 12 | 20 | 3/4" standard | 9" on-center | 250 | 50% Propylene Glycol | 320 | 2 | 120 | 18,000 | 1.6 to 2.2 | 13.9 | 3, 6 |
| 12 | 20 | 3/4" standard | 6" on-center | 250 | 50% Propylene Glycol | 480 | 2 | 120 | 18,000 | 1.6 | 13.9 | 3 |
| 20 | 20 | 3/4" standard | 12" on-center | 250 | 50% Propylene Glycol | 400 | 2 | 200 | Not in source | 2.2 | 13.9 | 7 |
| 20 | 20 | 3/4" standard (Inferred) | 9" on-center (Inferred) | 250 | 50% Propylene Glycol | 533 (Inferred) | 3 (Inferred) | 133.3 | Not in source | 2.2 | 13.9 | 8 |
| 2 | 50 | 3/4" standard | 3" on-center | 250 | 50% Propylene Glycol | 427.7 | 2 | 50 | 17,060 | 2.2 | 13.9 | 9 |
| 2 | 50 | 3/4" standard | 4" on-center | 250 | 50% Propylene Glycol | 328 | 2 | 50 | 134,42 | 2.2 | 13.9 | 9 |
| 2 | 10 | 3/4" standard | 3" on-center | 250 | 50% Propylene Glycol | 85.5 | 1 | 20 | 3,412 | 2.2 | 13.9 | 9 |
| Not in source | Not in source | 3/4" standard | 6", 8", 9", or 12" on-center | 250 | 50% Propylene Glycol | Not in source | Not in source | Not in source | Not in source | 2.2 | 13.9 | 10, 11, 12, 13, 14, 15, 16, 17 |
| Not in source | Not in source | 1/2" PEX-A | 6" on-center | 245 | 50% Propylene Glycol (Inferred) | 735 | 3 | 183.33 | 26,333 | Not in source | Not in source | 18, 19 |
| Not in source | Not in source | 3/4" PEX | 6", 9", or 12" on-center | 400–450 | 30–35% Propylene Glycol | Not in source | Not in source | Not in source | Not in source | Not in source | 13.9 (Inferred) | 18 |
The hard rules: individual loop lengths stay under 300 feet for 1/2-inch PEX and 450 feet for 3/4-inch. All loops on a manifold must be within 10-15% of each other to balance flow. The 50% propylene-glycol mixture is thicker than water, so pumps need higher head pressure. Get these wrong and the result is cold spots — the exact failure a heated driveway exists to prevent.
