Understanding mass flux density
Mass flux density is the mass flow rate spread over an area. Think spray dryers, packed towers, or membrane sheets: you already care about kg/s, but the useful number is often “per square foot” or “per square meter” so you can compare vendors who quote different duct sizes. Everything here routes through grams per second per square meter so you can slide between SI and imperial area without redoing the whole derivation.
How mass flux density conversion works
You already converted mass to grams and time to seconds on the top line. The only extra wrinkle is area: square feet must become square meters using the international foot, and square centimeters pick up a factor of 10⁻⁴ when you jump to meters squared.
Result = Value × (source as g/(s·m²) per unit) ÷ (target as g/(s·m²) per unit)
Quick check: moving from per hour to per second always introduces a factor of 3600. If you only changed area and the value looked 3,600 times off, you forgot the clock, not the ruler.
Units you will actually see on drawings
The dropdown is short on purpose. These labels are the ones that show up in duct schedules, HVAC submittals, and food-grade belt specs.
Gram per second per square meter
Factor: 1 (base)
Friendly SI flux. If you like kg/s/m² mentally, remember 1 kg/s/m² equals 1000 on this scale.
Kilogram per second per square meter
Factor: 1000 g/s per kg/s on the mass line, same m²
Heavy industrial loads on big cross sections. Nice when your mass flow is already in kg/s and you only need to divide by pad area once.
Kilogram per hour per square meter
Factor: spreads kilograms across an hour before landing in g/(s·m²)
Batch reports love per hour. Read the label twice: some sheets mix per minute for small lab coupons.
Pound per hour per square foot
Factor: avoirdupois pound and international square foot
Classic US coil and process air tables. One square foot is exactly 0.09290304 m², not a rounded “0.093” from a sticky note.
Pound per second per square foot
Factor: same foot, faster clock
Shows up when someone converted a mass flow to a very small footprint. Sanity check against your total lb/s mass balance before you trust it.
Gram per second per square centimeter
Factor: centimeter squared to meter squared costs 10⁴
Tiny coupons and lab-scale diffusion cells. Easy to overshoot by four orders if you eyeball cm² as “small” without converting.
Common mass flux conversions at a glance
Rules of thumb when you are on a ladder and only have your phone calculator.
| From | To | Pattern | Example |
|---|---|---|---|
| kg/(s·m²) | g/(s·m²) | × 1000 | 1 kg/(s·m²) = 1000 g/(s·m²) |
| kg/(h·m²) | g/(s·m²) | × 1000 ÷ 3600 | 3.6 kg/(h·m²) = 1 g/(s·m²) |
| lb/(h·ft²) | kg/(h·m²) | use exact ft² | Let the tool carry ft²→m² |
| g/(s·cm²) | g/(s·m²) | × 10⁴ | 1 g/(s·cm²) = 10⁴ g/(s·m²) |
| lb/(s·ft²) | kg/(s·m²) | lb→kg, ft²→m² | Heavier than it looks; check total mass flow |
| kg/(h·ft²) | g/(s·m²) | combine mass, time, area | Rare on US sheets; verify area basis |
How this differs from plain mass flow
Mass flow by itself answers how much material is moving. Flux density divides that story by how much floor or packing area carries the load. Two identical kg/s lines can wear very differently if one spreads across a wide plenum and the other pins all the flow into a patch the size of a notebook.
If someone hands you mass flow and area separately, multiply or divide carefully: flux is not automatically “flow divided by footprint” unless the footprint is exactly the active area the vendor used in the test report.
Mass flux FAQ
Which square foot definition is used?
International survey foot to meters. That lines up with mechanical drawings that label feet and inches on ductwork.
Can I multiply flux by area to recover mass flow?
Yes when the flux is uniform over that area and the area matches the measurement plane. Edge effects and maldistribution break the shortcut.
Why grams on the base row?
It keeps the numeric range pleasant when you jump between grams per second and kilograms per hour without juggling tiny decimals everywhere.
Is this the same as heat flux density?
No. Same pattern (something per area) but different physical quantity. Keep mass flux and watts per square meter in separate columns on your spreadsheet.