HVAC Sizing Calculator
Wrong-sized HVAC is the #1 cause of premature system failure. Get a right-sized AC tonnage and furnace BTU estimate for your Detroit Metro home based on 7 home characteristics and Climate Zone 5 baselines.
Don’t include unfinished basements, attics, or garages. If you’re not sure, look at your property tax record or zillow listing.
Drafts at windows? Cold floors over the basement? That’s poor. Recently sealed and added attic insulation? That’s good.
Based on Detroit Climate Zone 5
How We Got There
⚠ This is an estimate, not a real Manual J
Real HVAC sizing accounts for window orientation, specific R-values, duct losses, infiltration rate, and other factors a 7-question tool can’t measure. Use this to ballpark — but for an actual install, you need a proper on-site Manual J. Big Mike’s includes one with every free estimate.
Why Proper Sizing Matters
Oversized AC: Cools the room fast but doesn’t run long enough to remove humidity. You end up with a cold, clammy house and short-cycling that destroys the compressor.
Undersized AC: Runs constantly on hot days, can’t reach setpoint, burns electricity, wears out fast.
Oversized furnace: Heats then shuts off rapidly (short-cycling), big temperature swings, dust kicked around, parts wear out 2x faster.
Undersized furnace: Can’t keep up on the coldest Detroit nights, runs constantly, sometimes freezes pipes when it falls behind.
Want exact sizing for your home?
Big Mike’s does free in-home Manual J sizing for any homeowner considering a new system. We measure windows, check insulation, calculate ductwork — you get hard numbers, not a calculator estimate.
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Proper Sizing by a BBB Accredited Pro
Most HVAC contractors skip Manual J and “size by replacement” — meaning they install the same size as your old system without checking if it was right. That’s how homeowners end up with $7,000 oversized furnaces that short-cycle. Big Mike’s does the math every time, free, with every estimate.
View Our BBB Profile →Why Trust Big Mike’s With Sizing?
Family-owned, Authorized Dealer for Carrier, Lennox, and Bryant. 20+ years installing right-sized systems across Wayne, Macomb & Oakland County.
Free Manual J
Real on-site sizing included with every estimate — not just a calculator guess.
Authorized Dealer
Carrier, Lennox, Bryant. We have the full range to size your home properly.
No Pressure
If your existing system is right-sized, we’ll tell you. We don’t push replacements.
Rebate Eligible
Proper sizing is required for DTE rebates & federal 25C credit. We file the paperwork.
When you’re ready to turn this estimate into an actual install, here’s who Detroit Metro homeowners trust.
Communication was excellent throughout. Pricing was very fair compared to other quotes I received. Quality of work was top notch.
Amin and team were quick and thorough diagnosing and repairing the furnace. Response time was very timely. Will recommend and use again.
Pricing is the best out of all of them. Communication was great. Will recommend to friends and family.
Fast, reliable, and affordable! Arrived in under an hour, diagnosed the problem on the spot, sourced the parts themselves, and had me running again quickly.
Get an Exact Manual J
This calculator gets you in the ballpark. For the real number — including DTE rebate qualification — book a free in-home Manual J with Big Mike’s.
☎ (313) 398-0504