Methodology

Every formula, input, assumption and data source behind our tools, with the date we last verified it.

Last updated Oct 6, 2026

How we turn published rules into local numbers

Every calculator and dated guide on this site follows the same four rules. Use official data: climate figures come from NOAA’s 1991–2020 U.S. Climate Normals for the weather station nearest each ZIP code. Use published rules: the timing or sizing rule we apply comes from an extension service, a government agency or a manufacturer, cited on the page. Check the result: before a computed date or formula goes live, we compare it with what those sources publish for the same place or product, and report the difference. Show the limits: if the data is too far away or too uncertain, we say so instead of giving a number.

Matching your ZIP code to a weather station

Each of 41,488 ZIP codes is placed at its Census ZCTA center point (or the GeoNames point where no ZCTA exists) and matched to the nearest NOAA station with the data a tool needs. The median distance is 7 miles. We don’t publish a frost date if the nearest station with freeze records is more than 50 miles away, or if a 32°F night happens in fewer than half of years there. Elevation-based tools use the station’s elevation and let you enter your own.

Generator size calculator

Formula
Running watts = sum of ticked loads. Starting watts = running watts + the largest single (starting − running) surge. Recommended size = smallest standard size ≥ running × 1.05 ÷ derate factor; starting rating should be ≥ starting watts ÷ derate factor.
Derate factor
Portable: 1 − 0.035 × elevation (ft) ÷ 1,000 (Honda EG5000X owner’s manual). Standby: (1 − 0.03 × max(0, elevation − 600) ÷ 1,000) × (1 − 0.017 × max(0, hottest-month average high − 77) ÷ 10) (Generac RG Protector spec sheet, naturally aspirated models).
Inputs
Appliance list (running and starting watts, editable), generator type, optional ZIP code and elevation.
Defaults
Top of the running and starting ranges in Champion Power Equipment’s generator wattage chart.
Data source
Elevation of the NOAA station nearest the ZIP code (median 7 miles away) and its hottest-month normal maximum, NOAA NCEI U.S. Climate Normals 1991–2020.
Assumptions
Motors start one at a time; the 5% margin is our own allowance for small loads.
Limitations
Station elevation may differ from your home’s; turbocharged standby models derate only above 3,000 ft; your generator’s spec sheet wins. Not a load calculation; installation needs a licensed electrician.
Last verified
Oct 3, 2026

Pool salt calculator

Formula
lb = (target ppm − current ppm) × gallons × 8.34 ÷ 1,000,000; kg = (target − current) × liters ÷ 1,000,000. Too high: share of water to replace = (current − target) ÷ current.
Inputs
Pool volume, current reading, target ppm (3,000 / 3,200 / 3,400 / 3,600), bag size
Rain line
Salt left after pumping off R inches of rain = D ÷ (D + R), D = average depth in inches (saved by the pool volume calculator, else 60). Wettest-month ceiling = target × (1 − e−R/D), R = the wettest May–September monthly normal at the station nearest the saved ZIP.
Assumptions
99%+ sodium chloride, fully dissolved; rain that only replaces evaporated water removes no salt.
Data source
Weight of water 8.34 lb per US gallon (USGS Water Science School); salt ranges and charts: Hayward AquaRite Owner’s Manual 092538 Rev D (2019) and Pentair IntelliChlor Installation and User’s Guide P/N 520589 Rev R (2023); rainfall: NOAA NCEI U.S. Climate Normals 1991–2020.
Validation
Matches Hayward’s salt chart within 1 lb (267 lb for 10,000 gal and 1,067 lb for 40,000 gal from 0 ppm; 90 lb for 18,000 gal from 2,600 ppm) and Pentair’s 3,600 ppm chart within 0.1% (300.34 lb for 10,000 gal from 0 ppm; ours 300.2 lb).
Limitations
Readings vary between strips, electronic testers and titration (Pentair rates its sensor ±500 ppm); add in stages and retest after 24 hours of circulation.
Last verified
Oct 3, 2026

Pool volume calculator

Formula
Gallons = surface area (sq ft) × average depth (ft) × 7.4805. Rectangle: length × width. Round: π × (diameter ÷ 2)². Oval with straight sides: (length − width) × width + π × (width ÷ 2)². Freeform: sum of up to three rectangles or circles.
Constants
1 US gallon = 231 cubic inches (exact); 1 cubic foot = 1,728 cubic inches, so 7.4805 gallons; 1 gallon = 3.785 liters.
Assumptions
Average depth of a sloped floor = (shallow + deep) ÷ 2, exact for an even slope and slightly high for a hopper-shaped deep end.
Data source
NIST Handbook 44 (2026), Appendix C.
Limitations
Steps, benches and rounded floor edges are not subtracted; results are typically within a few percent for standard shapes.
Last verified
Oct 3, 2026

Frost date lookup

Method
Each ZIP code is matched to the nearest NOAA weather station that has freeze statistics, by straight-line (great-circle) distance from the ZIP code’s center. We show that station’s name, its distance from your ZIP and its elevation, so you can judge how well it represents your yard.
What the dates mean
Spring dates are the last 32°F night, fall dates the first. The 50% date is the median: in half of years the last spring frost comes later. At the 30% date there is a 30% chance of a later frost, and at the 10% date only a 10% chance, which is why we suggest planning tender transplants around the 10% date. Fall percentages work the same way in reverse (chance of a frost before the date). Hard-freeze dates use 28°F at 50%.
When we don’t show a date
If the nearest station with freeze records is more than 50 miles away (701 of 41,488 ZIP codes, mostly in Hawaii, Alaska and the territories), we publish no frost date. If a 32°F night happens in fewer than half of years at that station (1,557 ZIP codes, such as Phoenix and Miami), an “average” frost date is not meaningful, so we show how often frost occurs instead.
Other figures
Frost-free season is the station’s median growing-season length at 32°F. Heating and cooling degree days are annual normals at base 65°F. Monthly temperatures and precipitation come from the nearest station with complete monthly normals, which can differ from the frost station.
Data source
NOAA NCEI U.S. Climate Normals 1991–2020, annual/seasonal and monthly by-station files (version 1.0.1): 6,949 stations with freeze-probability normals and 6,719 with complete monthly temperature and precipitation normals.
Limitations
Normals describe a typical year from 1991 to 2020, not this year’s forecast. Valleys, hilltops, lakeshores and cities can shift frost dates by one to two weeks from the station’s, and a large elevation difference between your yard and the station matters most. For a cold night this week, check the National Weather Service forecast.
Last verified
Oct 3, 2026

Hardiness zone by ZIP code

Method
Your USDA Plant Hardiness Zone is read directly from the official 2023 ZIP-code table published by the PRISM Climate Group at Oregon State University, which produced the 2023 USDA map. We don’t compute or estimate zones ourselves.
Coverage
39,904 of our 41,488 ZIP codes are in that table. The rest, often PO-box or single-building ZIP codes, show no zone.
What a zone means
The zone is the average coldest winter night of 1991–2020 in 10°F bands, split into 5°F half-zones. It tells you which perennials survive the winter, not when to plant: use your frost dates for timing.
Data source
PRISM Climate Group, 2023 USDA Plant Hardiness Zone Map ZIP-code table (phzm_us_zipcode_2023.csv).
2012 comparison
The planting zone lookup also shows your ZIP code’s zone on the 2012 USDA map, read the same way from PRISM’s 2012 ZIP-code table (phzm_us_zipcode_2012.csv; 40,522 of our ZIP codes are in it). The change is counted in half-zones; the temperature band for any zone follows USDA’s definition: zone 1 starts at −60°F, each zone covers 10°F and each half-zone 5°F.
Last verified
Oct 6, 2026

Data sources