What it calculates
Estimate gallons captured from roof area and rainfall.
Rainwater Collection Calculator
Estimate rainwater captured from roof area and rainfall.
Planning and estimating aid only. Verify project requirements, manufacturer instructions, adopted codes, permits, and field conditions before performing regulated work.
Plumbing planning
How to Use This Calculator
Enter the measurements and job information you know, select Calculate, then use the result as a starting point for estimating, purchasing or project planning.
How the calculation works
Approximate gallons = roof area × rainfall in inches × 0.623 × collection efficiency.
What the result means
The efficiency factor accounts for losses such as first flush, splash and incomplete collection.
Use real project measurements
Start with the actual field dimensions or equipment values from the job. Run the calculator once with your measured quantity, then test a second scenario with your normal waste, routing or safety allowance. This makes the tool useful for both the takeoff and the customer estimate.
Contractor tip: Save the measurements that produced your estimate in the job notes so the quantity can be checked later if the scope changes.
Verify the final quantity
Use measured dimensions and actual fixture/product data whenever possible. Verify pipe sizing, drainage, venting, water-heater and installation requirements with applicable codes and manufacturer instructions.
Good practice: Compare the calculated quantity with plans, specifications, product packaging and current field conditions before purchasing material.