TryBuildCalcDuct Boot to Register Size Calculator (Fixed-Width Opening Sizing)
Size your register for a fixed duct boot width instantly.
🕒 Last updated: September 18, 2026
Inputs
ℹ️From a room-by-room load/airflow calculation — see the HVAC Airflow / Room CFM Calculator if you don't have this yet.
ℹ️Quiet uses a larger opening for less noise; Compact uses a smaller opening at the cost of more noise/draft.
ℹ️Sets the free area percentage — check a specific product's spec sheet once you've picked a model.
ℹ️Enter this only if your duct boot or framing bay width is fixed, to compute the required length.
ℹ️For context only — shows the feeding duct's cross-section next to the register/grille's required area.
Required Gross Face Area
38.4 sq in
At 500 FPM (Standard (Most Rooms)), standard louvered / bar-type core (75% free area)
At a 4 in fixed width, you need a length of at least 10 in (rounded up to the next even inch) to hit this required gross face area.
Opening & Velocity
Outlet type: Supply Register
Required airflow: 100 CFM
Velocity preset: Standard (Most Rooms)
Target face velocity: 500 FPM
Free Area & Sizing
Core type: Standard Louvered / Bar-Type (75%)
Required free area: 28.8 sq in (0.2 sq ft)
Required gross face area: 38.4 sq in (0.267 sq ft)
Assumptions Used
Required free area = required CFM ÷ target face velocity × 144. For a supply register, the target velocity comes from your chosen Quiet (400 FPM) / Standard (500 FPM) / Compact (650 FPM) preset — no single number is universally cited for supply registers, so this is a planning range, not one fixed industry standard. For a return grille, the target velocity follows ACCA's published distinction between filter grilles (300 FPM, to offset the filter media's own pressure drop) and non-filter/open grilles (500 FPM). Required gross face area divides the free area by your selected core type's free area percentage (75% standard louvered/bar-type, 60% perforated/egg-crate, 40% small/compact under ~8x8 in) — these are typical planning ranges; a specific product's own published free area should be used once you've picked a model. The optional required length (shown only when a fixed opening width is entered) rounds up to the next even inch, the common increment residential register/grille lengths are sold in. This is a planning estimate, not a substitute for a full ACCA Manual D design or a specific manufacturer's own sizing chart.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Register & Grille Sizing Calculator.
Fixed-width duct boot to register sizing
When a duct boot or framing bay width is already fixed (a common real-world constraint, especially in a retrofit), this calculator computes the required register length at that width, rounded up to the next even inch — the common increment residential register lengths are sold in.
This page defaults to a 4 in fixed opening width for 100 CFM — edit the inputs above to match your actual constraint.
Register & Grille Formula: How Is It Determined?
Two calculations run in sequence — required free area from CFM and velocity, then required gross face area accounting for the core type's own free area percentage.
Required Free Area
Required Free Area (sq in) = Required CFM ÷ Target Face Velocity (FPM) × 144
This is the minimum open area actually needed to pass your required airflow at your chosen target velocity. For a supply register, the target comes from your selected Quiet/Standard/Compact preset. For a return grille, it comes from ACCA's published 300 FPM (filter grille) or 500 FPM (non-filter grille) targets — the lower filter-grille target accounts for the filter media's own pressure drop.
Required Gross Face Area
Required Gross Face Area (sq in) = Required Free Area ÷ Core Type Free Area %
Free area is only the portion of a grille or register's visible face that's actually open — the rest is frame, louvers, or bars. This divides the required free area by your selected core type's typical free area percentage (75% standard louvered/bar-type, 60% perforated/egg-crate, 40% small/compact under ~8x8 in) to find the actual face size you need to shop for. Check a specific product's own spec sheet for its exact published free area once you've picked a model.
Required Length at a Fixed Width (Optional)
Required Length (in) = Required Gross Face Area ÷ Fixed Opening Width, rounded up to the next even inch
Only computed when you enter a known, fixed opening width (an existing duct boot or framing bay). Rounding up to the next even inch matches the common increment residential register/grille lengths are actually sold in.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Outlet type | Supply Register |
| Required airflow | 100 CFM |
| Velocity preset | Standard (Most Rooms) |
| Core type | Standard Louvered / Bar-Type |
| Fixed opening width | 4 in |
Step-by-Step Calculation
| Step | Calculation | Result |
|---|---|---|
| Required free area | 100 ÷ 500 × 144 | 28.8 sq in |
| Required gross face area | 28.8 ÷ 75% | 38.4 sq in |
| Required length @ 4 in width | 38.4 ÷ 4, rounded up | 10 in |
Therefore, this supply register passing 100 CFM at 500 FPM needs at least 38.4 sq in of gross face area with a standard louvered / bar-type core. At a fixed 4 in width, that means a length of at least 10 in.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.