Our methodology
How every calculator on this site is built.
Every sizing calculator on WhatSize Appliance is built on a publicly available standard, with every formula, multiplier, and lookup table documented. This page explains which standards we use, where the numbers come from, and how we keep everything current.
Last full review: June 30, 2026 • Next scheduled review: within 90 days
Standards reference
Each calculator traces to a published standard. The table below maps every standard to the calculator it drives, the specific section we use, and the date its application was last verified against the current edition.
| Standard | Calculator(s) | Key Section | Last Verified |
|---|---|---|---|
| AHAM AC-1 | Air Purifier | Section 4.2 (CADR test method) | June 30, 2026 |
| AHAM DH-1-2022 | Dehumidifier | Section 5 (capacity rating) | June 30, 2026 |
| DOE 10 CFR 430 | Water Heater | Subpart C (First Hour Rating) | June 30, 2026 |
| ACCA Manual J, 8th Ed | Mini Split | Simplified cooling-load calculation | June 30, 2026 |
| NEC Article 702 (NFPA 70) | Generator | Continuous-load + starting surge rules | June 30, 2026 |
| Manufacturer performance curves (Zoeller, Wayne) | Sump Pump | GPH vs TDH pump curves | June 30, 2026 |
Data sources
Every number in every calculator comes from a named, checkable source. The primary data sources we rely on across the site:
- Manufacturer spec sheets — Generac, Honda, Champion, Zoeller, Wayne, MrCool, Frigidaire, Levoit, Rheem, Bosch, and others. Published wattage, BTU, GPH, and CADR values are pulled from current spec sheets available on each manufacturer's website.
- DOE Appliance Energy Use Chart — Typical wattage and energy consumption ranges for major appliance categories, maintained by the Department of Energy.
- AHAM Verifide directory — Independently tested CADR values for air purifiers, maintained by the Association of Home Appliance Manufacturers.
- AHRI Directory — Certified performance ratings for HVAC equipment, including mini splits and heat pumps.
- ENERGY STAR product database — Efficiency-certified product specifications for dehumidifiers, air purifiers, water heaters, and mini splits.
Note: Wattage, BTU, and GPH figures used in our calculators are typical values derived from field measurements and published performance curves. They are not exact nameplate ratings, which tend to be higher than real-world operating consumption.
Formulas
Every formula on this site is deterministic — the same inputs always produce the same output. Below is the complete set of formulas that drive each calculator.
| Calculator | Formula |
|---|---|
| Generator | (∑ Running Watts × 1.25) + Largest Starting Surge → round to nearest standard size |
| Mini Split | Sq Ft × 25 × Climate Factor × Room Factor × Sun Factor × Ceiling Factor → round to nearest standard BTU |
| Dehumidifier | AHAM DH-1 Table lookup × Ceiling Factor + Basement(+5) + Laundry(+5) → round to nearest standard size |
| Air Purifier | Sq Ft × 0.67 × Concern Factor × Pollution Factor |
| Water Heater Tank | FHR ÷ Recovery Multiplier (Gas 1.7, Electric 1.1) → round to nearest standard size |
| Water Heater Tankless | ∑ Simultaneous Fixture GPM + 1.0 buffer |
| Sump Pump | Moisture Level + Pit Depth → HP class via manufacturer pump curve intersection |
Product recommendation criteria
When a calculator returns a result, we surface product recommendations that match the required size. The criteria we use to select products, in order of priority:
- Verified specifications first. The product must meet or exceed the calculator's output (BTU, CADR, GPH, watts, FHR). If a product doesn't clear the threshold, it never appears.
- Customer review volume and rating. Among qualifying products, we sort by review count and average rating from Amazon. Products with active CPSC recalls are removed immediately.
- Value (spec per dollar). Among products with comparable reviews, we prioritize the lower price. Our product recommendations are never sorted by commission rate — this is enforced by editorial policy.
- Monthly re-curation. Product lists are refreshed each month. Discontinued models, out-of-stock products, and superseded versions are removed; newly released qualifying products are added.
For the full conflict-of-interest policy and affiliate disclosure, see the Editorial Policy page.
Review schedule
We review and re-verify every component of the site on a fixed cadence. The schedule below is tracked internally and a "Last reviewed" date appears on every page.
| Content Area | Review Cadence |
|---|---|
| Calculator formulas | Reviewed quarterly against latest standard editions |
| Appliance wattage/BTU data | Reviewed every 6 months against DOE updates |
| Product recommendations | Re-curated monthly |
| FAQ content | Reviewed quarterly |
| Guide content | Reviewed every 6 months |
| Editorial policy | Reviewed annually |
| Last full review | June 30, 2026 |
| Next scheduled review | Within 90 days |
Limitations
Our calculators are accurate for typical residential use cases, but they have boundaries. Here is what they cannot do:
- Not a substitute for licensed contractor site evaluations. A calculator cannot inspect your ductwork, wiring, or structural conditions. For permanent installations — especially standby generators and mini splits — a licensed professional must perform a site assessment.
- Does not account for altitude, extreme climate micro-zones, or building envelope specifics. The calculators use average climate factors by region and standard room assumptions. Homes at high altitude, in extreme micro-climates, or with unusual insulation profiles will deviate from the model.
- Product prices change daily. Prices displayed alongside product recommendations are fetched at page-build time and may be stale. Always check Amazon for current pricing before making a purchase decision.
- For standby generator installations, a licensed electrician must verify per NEC 702.5. Our generator calculator sizes the unit; it does not perform the legally required load calculation for a permitted installation.
Corrections
We fix errors. If you find a mistake in any formula, recommendation, or piece of data, email [email protected]. We acknowledge every report within 2 business days and correct confirmed errors within 48 hours of confirmation. Significant corrections are logged publicly at the bottom of the affected page.
Questions about this methodology? Email [email protected] or visit the Contact page. To report a calculation error, write to [email protected].