Two features that used to be manual workarounds are now built directly into the AC BTU Calculator. You can type a room's length and width instead of pre-multiplying them yourself, and you can add several rooms into a single whole-house BTU and tonnage total instead of adding up separate results by hand. Here's exactly what changed and how to use each one.
What's new: length × width entry
What changedThe Sq Ft to BTU tab now has an "Enter length × width instead" toggle. Switch it on and two fields appear — Length and Width — instead of a single Room area field. Type both side lengths and the calculator multiplies them automatically, showing the resulting area right below the fields before it feeds into the same BTU formula as before.
Previously, getting a room's square footage into the calculator meant doing the multiplication somewhere else first — a phone calculator, a notepad, mental math — then typing the single result into one area field. That's a small step, but it's also a small place to make a rounding mistake or transpose a digit. The new toggle removes that step entirely: enter the two numbers you actually measured with a tape measure, and the tool does the arithmetic.
Example (hypothetical): say you measure a bedroom at 14 ft by 16 ft. Turn on the length × width toggle, type 14 into Length and 16 into Width, and the hint line below shows = 224 sq ft room area automatically — no separate multiplication needed before you hit Calculate.
Works with metric too — when the calculator's metric-units toggle is on, the Length and Width fields switch to meters right along with the rest of the tool, so the same dimension-entry workflow applies whether you measure in feet or meters.
What's new: the multi-room / whole-house planner
What changedA new multi-room planner section lets you add a row per room — each with its own settings — and sums every room's individual BTU/h requirement into one whole-house total, converted to tons automatically. You no longer have to run the calculator once per room and add the results up yourself.
This is the direct answer to a gap flagged in an earlier guide on this blog: room-by-room BTU calculation is more accurate than one blended house-wide estimate, because a sun-facing living room and a shaded bedroom of identical square footage rarely need identical cooling capacity. Doing that math by hand for four or five rooms, though, meant running the single-room calculator repeatedly and keeping a running total on paper. The planner does that summing automatically.
Each room row has its own remove control, so you can add exactly as many rooms as you're planning — and delete a row if you added a space by mistake or want to test a different room count before comparing central air or multi-zone mini-split quotes.
Illustrative example — four-room planner entry (hypothetical figures, for demonstration only)
| Room | Approx. sq ft | Approx. BTU/h |
|---|
| Living room | 280 | ~9,800 |
| Primary bedroom | 200 | ~6,200 |
| Second bedroom | 140 | ~4,300 |
| Kitchen | 150 | ~5,900 |
| Planner total | 770 | ~26,200 (≈2.2 tons) |
The planner produces that bottom-line total directly on the page — the same summed figure you'd previously have needed a separate spreadsheet or notepad to track across multiple single-room calculations.
Still a planning estimateThe planner total is a strong starting point for comparing central air or multi-zone quotes, but it still doesn't model shared ductwork, hallways, or whole-house infiltration the way a licensed contractor's full Manual J load calculation does. Treat it as the number you bring into that conversation, not the final spec.
Using both features together
The two features stack naturally: inside the multi-room planner, each room row is a normal room entry, so you can use length × width for any room where that's easier than knowing the square footage outright. Walk through a house with a tape measure, enter each room's two wall lengths as you go, and let the planner both compute each room's area and sum every room's BTU/h into the whole-house total in one pass — no separate multiplication step, no manual addition step.
Frequently asked questions
How do I enter length and width instead of square footage in the AC BTU calculator?
On the Sq Ft to BTU tab, switch on "Enter length x width instead." Two fields appear for the room's two side lengths; the calculator multiplies them automatically and shows the resulting area, which then feeds into the same BTU formula as manual square-footage entry.
Does the length x width entry work with metric units?
Yes. The calculator's metric toggle switches the length and width fields to meters alongside the rest of the tool, so the dimension entry works the same way whether you're measuring in feet or meters.
How does the multi-room BTU planner work?
The multi-room planner lets you add a row for each room in a home, entering that room's own square footage and settings. It sums every room's individual BTU/h requirement into one whole-house total and converts that total to tons, so you don't have to calculate each room separately and add the numbers up by hand.
Is the multi-room planner accurate enough to size a central air system?
It gives a solid planning-stage estimate by summing individual room calculations, which is more accurate than one blended whole-house average. It still doesn't account for shared ductwork, hallways, or whole-house infiltration the way a contractor's full Manual J load calculation does, so treat the total as a strong starting point for comparing quotes, not a final spec.
Can I remove a room from the multi-room planner after adding it?
Yes. Each room row in the planner has its own remove control, so you can add as many rooms as you're planning and delete any row if you added a space by mistake or want to try a different room count.