💨CFM Calculator
The CFM Calculator Toolset

One Airflow Engine, Three Layers of Control

A bathroom fan and a commercial kitchen do not need different tools — they need different amounts of the same tool. This page maps every layer of the cfmCalculator.net calculator: what it computes, which jobs it fits, and how the layers share one formula chain so the answers can never quietly disagree.

Open the Calculator Compare the Layers
Industrial ventilation fans and ductwork moving air through a buildingFrom room size to quiet duct — one engine end to endPhoto: Unsplash
Results on every keystroke
Inputs never leave your browser
Formulas shown for every step
CFM, m³/h & L/s together
No sign-up, no caps, no paywall
The Layers

What Each Layer Does

The layers are progressive disclosure, not separate products: the main panel is the duct and loss layer with the dials at their defaults, and the velocity and noise layer reads the result everyone else stops at. Start at whichever layer your job actually needs.

Layer 1 — The Main Panel

The CFM Calculator

Type a room, see an airflow. This layer answers the everyday question before you have finished measuring.

Open the main calculator
  • Describe the room by dimensions (length × width × height) or by floor area plus ceiling height
  • Set ACH from the built-in room-type reference, the quick picks, or your own number
  • Results update on every keystroke: base CFM in cfm, m³/h, and L/s at once
  • Scenario presets — Bathroom, Kitchen Hood, Garage, Grow Tent — fill ACH and room size in one tap
  • Switch imperial ↔ metric at any time; every input and output converts together
Layer 2 — Ducts & Losses

Friction, Elbows, and the Safety Factor

For installations where the free-air rating is a rumor — long runs, wall stacks, filtered systems.

See the loss math
  • Duct friction correction: +10% per 50 feet of run, shown as its own line item
  • Elbow loss: +5% per 90° bend, because every turn in the run costs real airflow
  • Rectangular duct converter: the ASHRAE equivalent-diameter formula for wall stacks and joist bays
  • Safety factor slider from ×1.0 to ×2.0 (×1.2 by default) for filters, caps, and flex sag
  • The final CFM is the number to shop against — not the bare-room figure most charts stop at
Layer 3 — Velocity & Noise

Air Velocity Checks and Fan Selection

A fan can be the right size and still sound wrong. The third layer keeps the duct quiet and the shopping list real.

Open the velocity chart
  • Live air velocity in FPM (or m/s) for round and rectangular ducts
  • Color-coded verdicts: quiet below ~600 FPM, normal to 900, noisy past 1,000, warning past 1,200
  • A capacity chart shows what each round duct diameter carries at each velocity
  • The fan-size recommender rounds your final CFM up to the next stocked blower
  • Warnings fire before you buy — not after the drywall closes
Under the Hood

The Chain Every Number Takes

All three layers run the same six-step chain. Knowing it means you can audit any result by hand — or pinpoint exactly which input to change when the fan aisle surprises you.

  1. 01

    Your Room

    Length, width, and ceiling height — measured, or floor area plus height from the plans.

  2. 02

    ACH

    Air changes per hour from the room type: 8 for bathrooms, 15 for hoods, 30 for grow tents.

  3. 03

    Base CFM

    Volume × ACH ÷ 60. The clean textbook answer every chart on the internet stops at.

  4. 04

    Losses & Margin

    Duct friction, elbow penalties, and your safety factor — each visible as its own step.

  5. 05

    Velocity Check

    The airflow is pushed through your duct size to predict noise before purchase.

  6. 06

    Fan Size

    The final CFM rounds up to the next stocked blower — the number for the shopping cart.

The full equations are printed on the formulas section of the home page — every result on the site traces back to them.

Reading Results

What Each Output Means — and Which to Plan Against

Six numbers leave the engine. Each answers a different question: one is for the room, one is for the duct, one is for the shopping cart — and one exists only so you can audit the rest.

OutputUnitsWhat It Tells You
Base CFMCFMVolume × ACH ÷ 60 — the textbook answer for the room alone, before any duct exists.
Loss Offset+%The duct friction and elbow penalties added together — the honesty line most calculators omit.
Final CFMCFMBase plus losses, times your safety factor — the figure to size the fan, the duct, and the budget against.
Metric Flowm³/h · L/sThe same airflow in the units the rest of the world buys in: ×1.699 for m³/h, ×0.4719 for L/s.
Air VelocityFPM · m/sHow fast the air moves through your duct — the number that decides whether the system whispers or whistles.
Next Fan SizeCFMYour final CFM rounded up to the next stocked blower size — the number for the shopping cart.

The engine computes the system you describe; the manufacturer’s fan curve governs what actually ships. Where the two disagree by more than your margin, check the duct length and bend count first — then use the formulas to work out why.

Presets

Scenario Presets the Calculator Speaks

Rooms come in types, not arbitrary decimals — so the calculator offers the common scenarios as one-tap chips instead of making you look up ACH. The presets below match how ventilation jobs are actually talked about on site.

Scenario Presets

🛁 Bathroom8 ACH · 4" duct🍳 Kitchen Hood15 ACH · 8" duct🚗 Garage6 ACH · 6" duct🌱 Grow Tent30 ACH · 10" duct

ACH Quick Picks

Bedroom 6Bathroom 8Kitchen 8Living room 7Laundry 8Office 7Restaurant kitchen 15Grow room 30

A preset fills ACH, safety factor, starter duct size, and typical room dimensions — then every field stays editable, because your room is the one that counts. For the full picture by room type, the ACH table on the home page covers 24 rooms from basements to spray booths.

A Walkthrough

One Bathroom, From Tape Measure to Shopping Cart

To see how the layers cooperate, follow a single realistic job the whole way through — a full bathroom, the single most common ventilation question the site answers.

The room measures 10 by 10 feet with 8-foot ceilings — 800 cubic feet of air. The Bathroom preset loads 8 ACH, and the main panel does the textbook math: 800 × 8 ÷ 60 gives a base of about 107 CFM. That is the number most charts on the internet would hand you, and it is honest — for a fan blowing into open air. The whole first layer of the tool took two inputs and about ten seconds.

The second layer is where this installation parts ways with the chart. The duct runs 25 feet to the roof with two 90° bends: the loss fields add 5% for the run and 10% for the elbows, lifting the requirement to about 123 CFM. The ×1.2 safety factor covers the wall cap, the future lint, and the flex sag, and the final figure lands near 147 CFM. The third layer keeps it quiet: through a 6-inch duct that airflow moves at a comfortable velocity in the normal band, and the fan-size recommender rounds 147 up to the next stocked blower — a 150 CFM unit, not the 110 CFM box the aisle instinct would have grabbed.

Notice what the outputs also handed you: the metric equivalents for a fan bought abroad (about 250 m³/h), the velocity reading that predicted the noise before purchase, and a loss breakdown you can show the inspector. One room, three inputs, and every downstream decision — fan, duct, budget, and expectations — has a defensible number behind it. That is the entire product philosophy of the site in one walkthrough: the math is yours to check, the margins are yours to set, and the velocity verdict refuses to flatter you.

Replay it in the calculator See more worked examples
By Project

Which Layer for Which Job

Six situations cover most of what people bring to the site. Each one names the layer to start from — and the section where the method is written out.

Bathroom Fans

8 ACH is the workhorse preset. Run a 10 × 10 ft room before you shop — the final CFM, not the box rating, decides whether the mirror ever clears again.

Open the section

Range Hoods

Cooktops are sized by capture: 100 CFM per linear foot of range. The Kitchen Hood preset loads 15 ACH and an 8-inch duct for the way hoods are really installed.

Open the section

Grow Tents

Heat, humidity, and CO₂ ride the airflow — 30 ACH by default. Add the filter and flex losses before buying, because the paper answer is always the flattering one.

Open the section

Supplier Quote Check

Run the room before the quote arrives. When their fan size and yours disagree, the loss breakdown gives you the shared arithmetic to resolve it.

Open the section

Duct Route Planning

Read the velocity badge: under 900 FPM is quiet, past 1,000 whistles. Step up one duct size and the same fan runs silent — the cheapest fix in ventilation.

Open the section

Learning the Math

Change one input at a time and watch which outputs move. The live engine is fast enough to teach ventilation intuition by experiment.

Open the section
The Honest Comparison

Why Not Just Estimate Another Way?

Every method of sizing a fan on paper works — within its limits. This is where each one runs out, including ours. The right choice depends on the stakes: a half bath forgives a bad guess; a grow tent full of flowering plants does not.

MethodWhere It ShinesWhere It Breaks
The 1 CFM/ft² rule of thumbInstant, good for whole-house sanity checksIgnores ceiling height, room type, ducts, and noise entirely
A hand-built spreadsheetFlexible once it existsFormula drift, no unit switching, breaks when the duct route changes
Fan store chartsClose to the products they sellFree-air ratings only — no losses, no velocity verdict, no margin
This calculatorVolume × ACH × losses × margin × velocity, live and in one placePlanning-grade by design — the manufacturer's fan curve still wins
Ventilation plans and duct fittings laid out on a planning table beside a laptopPick the method the stakes deservePhoto: Unsplash

Specialty Calculators

🌀CFM to FPM CalculatorConvert CFM to FPM and FPM to CFM for any round or rectangular duct — plus m/s, MPH and km/h, with the velocity-pressure and anemometer methods.🔄CFM Conversion CalculatorConvert CFM to m³/h, CMH, CMM, L/s, LPM, m³/s and CFH instantly with the full conversion factor table — imperial to metric airflow units.🔥CFM to BTU CalculatorConvert CFM to BTU and BTU to CFM with the 1.08 ΔT formula — furnace, boiler, kW duct heater and cooling airflow sizing in one tool.⚙️Fan Performance CalculatorCalculate fan CFM from RPM with the affinity laws — static pressure, BHP, in. w.c. and the fan-curve operating point for existing fans.🍳Range Hood CFM CalculatorFree range hood CFM calculator: size kitchen hoods by cooktop linear feet or kitchen ACH, island vs wall rules, commercial hood-area CFM, 600 CFM duct and makeup air.☀️Attic Fan CFM CalculatorAttic fan CFM calculator: size attic exhaust at 0.7–1.0 CFM per sq ft with roof-color and insulation adjustments, verify by air changes, and balance net free area (NFA).🔧Air Compressor CFM CalculatorAir compressor CFM calculator: total your tools' CFM at 90 PSI with duty-cycle factors, verify output with the tank-fill test, convert CFM across pressures, and match HP.🌡️SCFM to CFM CalculatorSCFM to CFM calculator with pressure and temperature correction — compare SCFM, CFM (ACFM) and Nm³/h baselines, plus mass-flow (lb/min) conversions for real compressor specs.🌀Blower CFM CalculatorBlower CFM calculator: size any blower from wheel diameter, width and RPM with per-type coefficients, compare centrifugal vs axial, and match static pressure.🌱Grow Tent CFM CalculatorGrow tent CFM calculator: size 2×2 to 4×8 tents by air changes, then stack carbon filter, grow-light heat and duct losses — with AC Infinity fan stage matching and CO₂ notes.❄️CFM per Ton CalculatorCFM per ton calculator: convert tons to CFM at 350, 400 or 450 CFM per ton for AC and heat pumps, split airflow room by room, and pick the right rate for your climate.🪵Dust Collector CFM CalculatorDust collector CFM calculator: match machine CFM demands, keep duct velocity at 4,000–4,500 FPM, size the fan from CFM and static pressure, and check baghouse air-to-cloth ratio.🎨Paint Booth CFM CalculatorPaint booth CFM calculator: size cross-draft and downdraft spray booths from face velocity, then match lab fume hoods at 80–100 FPM over the sash opening — fans and ducts included.🌫️Air Purifier CFM CalculatorAir purifier CFM calculator: match CADR to room size with the 5 ACH rule, convert air scrubber CFM for construction dust, and check HEPA filter media area at 250–500 FPM.💧Evaporative Cooler CFM CalculatorEvaporative cooler CFM calculator: size swamp coolers at 20–40 air changes per hour (2–3 CFM per sq ft) with dry-climate corrections, window vs whole-house picks and duct notes.🌬️Air Handler CFM CalculatorAir handler CFM calculator: split supply vs return air, check filter face velocity at 300–625 FPM, size return grilles at 350–500 FPM, and see CFM per ton.🏎️Engine CFM CalculatorEngine CFM calculator: size carburetors from CID and RPM with VE, correct for boost pressure, match throttle bodies up to 102 mm, and pick radiator fan CFM.🌿Greenhouse CFM CalculatorGreenhouse CFM calculator: summer cooling at one exchange per minute, winter 2–3 ACH, HAF circulation fans, and evaporative pad pairing for any size greenhouse.🚿Bathroom Exhaust Fan CFM CalculatorBathroom exhaust fan CFM calculator: size by room volume at 8 ACH, 1 CFM per sq ft, or the 50 CFM fixture rule — plus duct corrections and sone noise.🔁Air Changes per Hour CalculatorConvert air changes per hour to CFM and back with the ACH formula — room-by-room ACH standards, worked examples, and why delivered airflow beats nameplate.📐CFM to Square Feet CalculatorTurn CFM into square feet: coverage area from 700, 1500 or 3300 CFM at 0.5–3 CFM per ft², with ceiling-height corrections and device coverage charts.

Questions About the Calculators

Is there more than one calculator on the site?▼

One engine, three layers. The main panel handles the everyday volume × ACH question instantly; the duct and loss layer adds friction, elbows, the rectangular-duct converter, and the safety factor; the velocity and noise layer checks the duct and recommends the fan size. All three run the same math, so answers never disagree between layers.

Which number should I buy the fan against?▼

The final CFM — base, plus duct and elbow losses, times your safety factor. The box rating is free-air performance; real installations lose flow to filters, wall caps, and flex duct, which is exactly why the loss and margin stages exist. For duct selection, use the velocity output instead — it keeps the run below the noise bands.

Why does the calculator ask for the duct at all?▼

Because the room and the duct are one system. A 107 CFM bathroom fan pushing through 25 feet of duct with two bends needs roughly 15% more blower than the bare-room figure — and squeezing it through a 4-inch duct instead of a 6-inch one turns the difference into noise. Entering the duct makes the recommendation match the installation, not the brochure.

Does ACH really change the answer that much?▼

It is the single biggest lever. The same 800 ft³ room needs 40 CFM at 3 ACH and 400 CFM at 30 ACH — a tenfold swing for identical geometry. That is why the calculator ships with a room-type ACH table and scenario presets instead of a single default, and why the field always accepts your own number.

Can I plan a whole house or multi-room system?▼

Run each room once, note its final CFM, and size its duct from the velocity chart — the per-room outputs give you everything needed to line results up in your own takeoff sheet and sum them. For central systems, whole-house numbers follow the same math at roughly 1 CFM per square foot, but duct design for a central air handler belongs with a professional.

How current is the ACH data?▼

The table uses widely published ventilation guidance for each room type — bathrooms 6–8, bedrooms 5–6, commercial kitchens 14–30, grow rooms 20–40, and the rest. These are planning conventions, not physical constants; what changes between projects is the code adopted in your jurisdiction and the way the space is actually used, which is why the ACH field is editable and why the engineer of record wins.

Are more calculators coming?▼

The roadmap is driven by field requests: a static-pressure calculator, altitude and temperature corrections, a printable fan schedule, and multi-room project mode. Corrections to the existing engine always outrank new tools.

Run Your First Room Now

Type a room size, watch the CFM appear, then add the losses and read the velocity verdict before the fan leaves the shelf. Free, private, and instant — nothing to sign up for and nothing to install.

Open the CFM Calculator Read the Guides First