Free · No sign-up · Every step of the math shown
Know the size before you buy.
One question per tool, answered with the published chart or method behind it. Mini splits, generators, pools, water heaters, insulation and more. If a number isn't published, the tool says so instead of guessing.
How every tool works
- The size comes from a government, code or maker's published source, quoted on the page.
- Every step of the math is shown, line by line.
- Product specs come from the maker. Blank means they don't publish it.
- When the published method runs out, the tool says so and points you to a pro.
The tools
Each one answers a single question you ask right before an expensive purchase. Your answers stay in the page's address, so you can share the result.
- SplitSizerWhat size mini split do I need?Tool 01 · Mini split
- GenSizerWhat size generator do I need?Tool 02 · Generator
- PoolSizerHow many gallons is my pool?Tool 03 · Pool gallons, pump, heater
- TanklessSizerWhat size tankless water heater do I need?Tool 04 · Tankless water heater
- TankSizerWhat size water heater do I need?Tool 05 · Tank water heater
- AtticSizerHow much attic insulation do I need?Tool 06 · Attic insulation
- DehuSizerWhat size dehumidifier do I need?Tool 07 · Dehumidifier
- ACSizerWhat size window AC do I need?Tool 08 · Window and room AC
- SoftenerSizerWhat size water softener do I need?Tool 09 · Water softener
- WattSizerWhat size power station do I need?Tool 10 · Portable power station
Where the numbers come from
Every method and source →| No. | Tool | The size comes from |
|---|---|---|
| 01 | SplitSizer | ENERGY STAR's room air conditioner sizing chart, with its shade, sun, people and kitchen adjustments. |
| 02 | GenSizer | Champion Power Equipment's published sizing method and wattage chart. Safety lines from the CPSC and the CDC. |
| 03 | PoolSizer | Geometry for gallons, two published pool codes for pump turnover, and energy.gov's heater formula. |
| 04 | TanklessSizer | The U.S. Department of Energy's flow-rate and temperature-rise method. |
| 05 | TankSizer | The U.S. Department of Energy's peak-hour worksheet and first-hour rating. |
| 06 | AtticSizer | ENERGY STAR's recommended R-values, a national lab's county climate zones, and each maker's coverage chart. |
| 07 | DehuSizer | ENERGY STAR's dehumidifier sizing chart and its list of certified models. |
| 08 | ACSizer | ENERGY STAR's room air conditioner chart and its list of certified models. |
| 09 | SoftenerSizer | The formula in North Dakota's water softener code, EPA water-use figures and each maker's spec sheet. |
| 10 | WattSizer | energy.gov's appliance energy formula and sample wattages, checked against each maker's published capacity. |
Why these exist
Bigger isn't better. ENERGY STAR warns that an oversized air conditioner costs more, wastes energy and can leave a room damp. energy.gov says an oversized water heater costs more to buy and to run.
These tools use the published chart or method instead, show the math, and tell you when the honest answer is to have a pro run the numbers.
How they're paid for
Some tools show "check price" buttons. Those are affiliate links: if you buy through one, Foundry NW may earn a commission at no extra cost to you. A button never changes the size a tool recommends.
Shorts on YouTube
Each Short takes one "what size do I need?" question and works it with real numbers from the published source. Then you size your own with the tool.