ThermoMath

Homeowners · Contractors · Students

HVAC calculators
that show the math.

Size your AC, furnace, or ductwork with confidence. Instant results as you type — no account, no login, every formula explained.

Featured Tools

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BTU Calculator

Calculate the BTUs needed to heat or cool any room. Accounts for room size, ceilings, windows, climate zone, and occupants.

↳ 250 sq ft · hot climate → 8,750 BTU

HVAC Load Calculator

Whole-home simplified Manual J estimate. Enter square footage, insulation, climate zone, and window ratio to get cooling and heating loads.

↳ 2,000 sq ft · mixed climate → 3.5 tons

AC Size Calculator

Quick square footage to AC size lookup. Enter your floor area and climate, get a BTU and tonnage recommendation with a reference table.

↳ 1,200 sq ft · moderate → 2 ton unit

Furnace Size Calculator

Find the right furnace BTU output for your home. Includes climate zone, insulation quality, and efficiency (80% vs. 96% AFUE) recommendations.

↳ 1,800 sq ft · cold zone → 90,000 BTU

CFM / Airflow Calculator

Determine the airflow in cubic feet per minute required for a space. Set your room dimensions and air changes per hour target.

↳ 400 cu ft · 6 ACH → 40 CFM needed

SEER Savings Calculator

Estimate annual energy savings when upgrading to a higher SEER system. See payback period and lifetime CO₂ reduction.

↳ SEER 10 → SEER 18 · 3 tons → $380/yr saved

How it works

01

Type, see results immediately

Every input updates the output the moment you type. No submit button, no page reload — numbers appear as fast as you think.

02

Show the math

Each tool explains the formula and the assumptions behind it. Know why right-sizing matters, not just what number to buy.

03

Fully local, always free

All calculations run in your browser. Your inputs stay on your device — no account, no paywall, no subscription required.

FAQ

Are these results accurate enough for an actual HVAC purchase?
These tools implement standard industry sizing formulas and are appropriate for initial planning, budgeting, and cross-checking contractor quotes. For a code-compliant installation, a licensed HVAC professional should perform a full Manual J calculation. Think of these as informed starting points, not final specifications.
Do I need an account or email address to use any tool?
No account, no email, no sign-up. Every calculator on this site is fully accessible the moment you open it. Results are never gated behind a form.
What's the difference between BTUs and tonnage?
BTU (British Thermal Unit) measures heat energy — specifically, the energy needed to raise one pound of water by 1°F. Tonnage is a shorthand unit: 1 ton of HVAC capacity equals 12,000 BTU/hour. A 2-ton AC can remove 24,000 BTU of heat per hour. All our calculators output both units so you can match equipment specs directly.
Can I use these on a job site from my phone?
Yes. Every tool is mobile-first and touch-friendly. The layouts adapt to any screen from a small phone to a desktop monitor. Since all calculations run locally in the browser, they keep working even on a slow or spotty job site connection.
What is a SEER rating and why does it matter?
SEER (Seasonal Energy Efficiency Ratio) measures how efficiently an AC or heat pump cools your home over an entire season. A higher SEER means lower electricity consumption per BTU of cooling. As of 2023, federal minimum standards require SEER 14–15 depending on region, while premium units reach SEER 20–26. Upgrading from a SEER 10 to SEER 18 system can cut cooling costs nearly in half.
How is the BTU Calculator different from the HVAC Load Calculator?
The BTU Calculator is room-level — great for sizing a window unit, mini-split, or portable AC for a single space. The HVAC Load Calculator works at the whole-home level, accounting for total square footage, building orientation, insulation quality, and construction year to produce a system-wide cooling and heating load. Use the BTU Calculator for a quick room check; use the HVAC Load Calculator when sizing central air or a new furnace.
I think a result looks wrong. What should I do?
Send a note through the Contact page with the tool name, your exact inputs, and the result you expected. All formulas are validated against ASHRAE and ACCA standards — genuine errors get fixed promptly.