Methodology
How our formulas are sourced
Every calculator is built from a specific, citable formula — not a general approximation. Here’s exactly how we source and verify them.
Formula sources by category
Pool & Spa Chemistry
Association of Pool & Spa Professionals (APSP) guidelines and CDC Model Aquatic Health Code (MAHC). Chlorine dosing formulas follow the MAHC free chlorine to cyanuric acid ratio recommendations.
HVAC & Energy
ASHRAE standards and U.S. Department of Energy (DOE) residential energy guidelines. Heating and cooling load calculations follow ASHRAE Manual J procedures.
Solar & Electricity
U.S. Energy Information Administration (EIA) rate data and National Renewable Energy Laboratory (NREL) solar irradiance methodology. Payback calculations use simple payback period, documented on each page.
Mortgage & Finance
Standard amortization formulas as published by the Consumer Financial Protection Bureau (CFPB). Results are mathematical outputs of the entered inputs and do not constitute financial advice.
Lawn, Garden & Irrigation
USDA cooperative extension service guidelines and manufacturer specifications for fertilizer and chemical application rates.
What we document on each page
The formula
Plain-English explanation of how it works
Worked example
Real numbers showing the calculation step by step
Authoritative source
Outbound link so you can verify the math yourself
Edge cases
Where the formula behaves differently or needs adjustment
Reporting an error
If you believe a formula is incorrect, produces unreasonable results, or is missing a material edge case, please contact us with the calculator name, the inputs you used, the result you received, and what you believe the correct result should be. We investigate every report.
This methodology page is reviewed quarterly. Last reviewed: May 2026.