Add a terminal design/measured flow schedule to identify the index (least-favoured) terminal, each terminal's ratio-to-index target flow, and a suggested work order: CIBSE Commissioning Code A / BSRIA BG49 proportional method.
A commissioning aid, not an automated control system, re-run after each round of damper adjustment with fresh readings.
For design guidance only. Always verify with a qualified engineer.
About this proportional balancing aid calculator
This free proportional balancing aid calculator supports the proportional method of balancing an air or water distribution system: enter a schedule of design and measured flows for each terminal, and it identifies the index (least-favoured) terminal, gives each terminal a ratio-to-index target flow, and suggests a work order. It is aimed at commissioning engineers and M&E contractors balancing diffusers, radiators or fan-coil branches to the CIBSE Commissioning Code A / BSRIA BG 49 proportional approach. It is a commissioning aid, not an automated control system, re-run it after each round of damper or valve adjustment with fresh readings. Everything runs in your browser. Nothing is uploaded.
How proportional balancing is calculated
Each terminal’s %-of-design = (measured ÷ design) × 100. The index terminal is the one with the lowest %-of-design, the least favoured, which is never trimmed. Every other terminal’s target flow = design × (index %-of-design ÷ 100), so you trim each one down toward the index rather than opening the index up, and the trim needed = measured − target. The suggested work order is ascending %-of-design, following Code A’s principle of starting at the extremities and working back toward the fan or pump. Advisory tolerance bands of ±10% per terminal and ±5% on the system total are a cross-industry convention (ASHRAE / TAB specifications), applied with a tiny epsilon so exact boundary values are forgiven.
Frequently asked questions
What is the index terminal?
The index terminal is the one running furthest below its design flow, the lowest %-of-design in the schedule. And it is the reference the rest are balanced against. Because it is already the least favoured, you never trim it up; you close down the others until each matches the same proportion of design, then increase the fan or pump to lift the whole system to duty.
Are the ±10% and ±5% tolerances from CIBSE?
Not directly. CIBSE Commissioning Code A’s own tolerance table is cumulative and scales with outlet count rather than being a single flat figure, so the calculator uses the widely quoted cross-industry bands, ±10% per terminal or branch and ±5% on the main total, as in ASHRAE / TAB specifications, as an advisory check only.
Do I re-run it as I balance?
Yes. The proportional method is iterative: adjust the dampers or valves, take a fresh set of measured flows, and re-enter them. The index terminal and targets can shift as you go, so the tool is a live aid you re-run each round. It does not replace on-site judgement or a commissioning specialist.