AC Ducting Length Calculator

Works out the total duct run length and an estimate of the fittings for a ducted air-conditioning layout. It runs entirely in your browser and shows the working, so you can check every step against your own figures.

How the ac ducting length calculator works

The total duct length is the main trunk plus a branch to each room. The fittings estimate allows two per room — a branch take-off and a register — plus the bends you enter. Short runs and few bends keep the static pressure down, so the fan moves air efficiently and quietly.

What you enter

Enter the main run, the branch run length, the room count and bends; the calculator returns the total duct length and fittings. Every field is editable, and the result recalculates the moment you change a value.

Reading the result

Use the length to order flexible or rigid duct, and the fittings count for take-offs and grilles. Every bend adds resistance, so a layout that keeps the trunk short and the branches direct performs better than a longer, twistier one.

Safe by design

This tool sizes the cooling load and running cost only. Refrigerant handling and the electrical connection are regulated work for a qualified F-Gas and electrical engineer.

Assumptions and limits

This estimates length and fittings from a simple trunk-and-branch layout. It does not size the duct diameter for airflow or pressure drop, which depends on the unit’s output and is part of the ductwork design.

Using it with the rest of BuildMetricLab

This calculator is one step in planning the job. Pair it with the related tools in this category and across BuildMetricLab to go from a single figure to a full costed plan — a sizing figure feeds a cost estimate, a cost estimate feeds a project budget, and each result carries straight into the next tool. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.