AC Ducting Length Calculator

A ducted air-conditioning system gets measured before it gets priced. This calculator totals the supply ductwork in a trunk-and-branch layout, separates the run that governs fan performance from the quantity that governs the material order, and counts the fittings that join the two. It runs entirely in your browser and shows the working, so every figure can be checked against your own measure.

How the AC Ducting Length Calculator works

A trunk-and-branch layout has one main duct leaving the indoor unit, with a branch dropping off it to serve each room. The total supply duct is the trunk plus one branch per room, so len = main + branch × rooms. A 10 m trunk with 4 m branches to three rooms comes to 22 m of duct.

That total is an ordering quantity, not the length the fan works against. Air leaving the unit travels one path to one register, so the run that sets the static pressure is the trunk plus a single branch — the critical path. The calculator reports it separately as the longest single run, 14 m in the same example.

Fittings resist airflow out of all proportion to their size, so duct design converts each one into an equivalent length of straight duct and adds it to the measured run. That is what the effective length row does: it takes the critical path and adds the bends sitting on that path, each multiplied by the equivalent length allowed per bend. Bends elsewhere in the layout are counted as fittings to buy, but they do not load this path and are left out of it. Two bends on that path at 7.5 m each add 15 m, taking a 14 m critical path to roughly 29 m effective. Duct is sized from that effective figure rather than from a tape-measure length.

What you enter

Six figures: the main trunk run, the branch run to a typical room, the number of rooms served, the number of bends in the whole layout, how many of those bends fall on the longest run, and the equivalent length allowed per bend. Every field is editable, and the result recalculates the moment a value changes.

The last of those is a variable rather than a constant, which is why it is an input. A smooth, large-radius sheet-metal elbow costs a few metres of equivalent length; a tight elbow in flexible duct can cost several times as much, because the corrugated wall and the compressed inner radius both add friction. The 7.5 m default sits mid-range among the 90° elbow entries in the ACCA Manual D fitting tables, which spread from smooth large-radius rigid elbows at the bottom to compressed flexible ones several times higher. Mid-range is not low, as the next section shows. A figure from the duct manufacturer’s own fitting tables replaces it directly.

Reading the result

The headline total is what goes on the order: the metres of trunk and branch duct the layout consumes. It answers a purchasing question, and on its own it says nothing about how the system will breathe.

The two length rows below it answer the performance question. A layout with a short critical path and few bends has a lower effective length, and a lower effective length leaves the blower less resistance to overcome. What that buys depends on the blower. A fixed-speed unit moves more air at the same speed. A variable-speed one holds its target airflow and drops its speed to do it, which usually shows up as less noise and less power drawn for the same air delivered. Two systems can total identical duct and behave nothing alike: 22 m spread across three short branches is not the same system as 22 m in one long run snaking to a far bedroom.

Why the fitting count matters

Duct is bought by the length; fittings are bought one at a time, and on a domestic system they account for a large share of both the material cost and the labour. The count allows a branch take-off and a register for each room, one collar where the trunk leaves the plenum, and the bends entered — so three rooms with four bends comes to eleven separate items, before boots, dampers or flexible connectors are added to the order.

The same count feeds back into the effective length, and the material decides how heavily. Rigid duct and flexible duct carry very different friction rates, and so do their fittings: a smooth large-radius rigid elbow sits at the low end of the equivalent-length tables, a tight elbow in compressed flexible duct at the high end, several times the rigid figure. Every fitting is worth far more than it measures, whichever material it is in. How much more is a property of the layout, not a constant: the ratio is one plus the total bend allowance on that path divided by the path length, so it climbs with the bend count and eases as the run gets longer. On the layout this page opens with — a 14 m path carrying two bends at 7.5 m each — the effective length lands at 2.07× the measured one, where a single bend on that path would give 1.54× and four would give 3.14×. An identical fitting count therefore means two different things in the two materials, which is why the per-bend allowance is asked for rather than assumed.

Certified and licensed work

Refrigerant handling and the electrical connection sit outside a quantities calculator, and outside what an estimator signs off. Both are certified and licensed work: a technician certified to handle refrigerant under the federal ozone-depleting-substances rules, and a licensed electrician working to the Canadian Electrical Code for the supply.

What effective length means

Effective length — equivalent length in some references — is the answer to a single question: how long a plain straight duct would have to be to resist airflow as much as this run does with its fittings in place. A bend is a few centimetres of duct that behaves like several metres of it, so the figure runs well above anything a tape measure produces — the worked example above is a case of it. That is the point of it: duct is sized from resistance, and resistance is what this figure carries.

The number here covers the critical path and the bends on that path. Two things it leaves out are worth naming. The branch take-off and the register at the end of the run each carry an equivalent length too — in ACCA Manual D terms a take-off can exceed an elbow — and neither is included, so the true effective length of the path is higher than shown. Return-air ductwork is absent for the same reason, and a full duct design adds it to the same total. The figure is a floor, not a ceiling.

Assumptions and limits

The model is a single supply trunk with one branch per room, all branches the same length and measured flat. Real branches differ, and the two figures then want different inputs: the longest branch gives the critical path that governs pressure, while an average one gives the truer material total. Vertical drops into a ceiling void or a riser are not separated out, and no slack is added — flexible duct is normally ordered with spare length so it can be pulled without kinking, and that allowance sits outside this figure.

Return-air ductwork is not modelled. A ducted system has a return path back to the indoor unit, and in a full duct design the return run and its fittings form part of the total effective length; only the supply side is counted here. The plenum collar is included in the fitting count, but transitions, plenum boxes and grille boxes are not, and no fitting other than a bend contributes to the effective length.

Nor does this size the duct itself. Diameter follows from the airflow each branch has to carry and the friction rate the design allows, both of which come from the unit’s output and the room-by-room load — that is ductwork design, and it sits downstream of a measure like this one.

Using it with the rest of BuildMetricLab

A duct measure is one line in an air-conditioning job. The cooling load calculator sizes the indoor unit the trunk leaves, the multi-room sizing calculator settles how many branches there are before any duct is measured, and the running cost calculator puts a figure on what the finished system draws. Every BuildMetricLab tool works the same way, so a figure carries straight from one to the next.