Mass Flow Rate Converter — kg/s, kg/h, lb/h, tonne/h

The mass flow rate converter above moves a value between kilograms, grams, pounds, ounces and tonnes per second, minute or hour. Enter a number, pick two units, and the conversion appears live, with the same rate displayed simultaneously in the four units most often used on process datasheets and instrument calibration certificates.

Arb Digital publishes this within a free engineering converter library. Mass flow is the quantity that matters wherever a process depends on how much material is actually moving rather than how much space it occupies — combustion, chemical dosing, steam systems, compressed gas, and anything where temperature or pressure changes the density of the stream while the mass stays the same.

Mass Flow Is Not Volumetric Flow

This distinction is the whole reason this page exists separately, so it comes first. Mass flow rate measures the mass of material passing a point per unit time, in kilograms per second or pounds per hour. Volumetric flow rate measures the volume passing a point per unit time, in litres per minute or cubic metres per second. They are different physical quantities with different dimensions, and no unit converter can turn one into the other on its own.

If you have litres per minute, cubic metres per hour, gallons per minute or cubic feet per second, you want our flow rate converter, which handles volumetric flow. This page handles mass only. The two unit lists are kept strictly separate here so that a cross-quantity conversion is structurally impossible rather than merely discouraged — you cannot accidentally select kg/s in one box and L/min in the other, because L/min is not on this page.

Bridging the two requires density: mass flow = volumetric flow × density. A stream of water at 1,000 kg/m³ flowing at 0.001 m³/s carries 1 kg/s of mass. The same volumetric flow of air at roughly 1.2 kg/m³ carries only 0.0012 kg/s. That thousand-fold difference for the identical volumetric number is exactly why the two quantities must not be conflated. Use our density converter to get the density into consistent units before multiplying.

Why Gases Make the Distinction Critical

For liquids at ordinary conditions, density barely moves, so volumetric flow is a reasonable proxy for mass flow and the industry often treats them interchangeably without harm. For gases that assumption collapses. A gas doubles in volume when its absolute pressure halves, and expands with temperature, so the same mass flow produces wildly different volumetric readings depending on where in the system you measure.

This is why gas flow is so often quoted in "standard" or "normal" volumetric units — standard cubic feet per minute, or normal cubic metres per hour. Those are not really volumes at all; they are mass flows disguised as volumes by referencing a fixed temperature and pressure. The trap is that "standard conditions" is not one thing: different bodies and different industries use different reference temperatures, so two SCFM figures from two sources may not describe the same mass flow. If you are converting a standard-condition figure, find out the reference conditions before trusting it. This converter deliberately excludes those pseudo-volumetric units because their definitions vary by source, and a converter that silently picked one would be misleading.

How to Use It

1. Enter the mass flow rate from your instrument, datasheet or process calculation.
2. Choose the source unit — kg/h for most European process work, lb/h for American practice.
3. Choose the target unit your calculation or report needs.
4. Read the result, which updates on every keystroke; the Convert button stays for keyboard and screen-reader users.
5. Use Swap units to reverse the direction of the conversion instantly.

The Formula: How Mass Flow Conversion Is Calculated

All conversions pivot through the kilogram per second:

result = value × factor(from) ÷ factor(to)

The mass factors are exact by international definition. The international avoirdupois pound is exactly 0.45359237 kilograms, agreed in 1959 and recorded in NIST Special Publication 811. From that one definition everything else follows: an ounce is a sixteenth of a pound, or 0.028349523125 kg exactly; the US short ton is 2,000 pounds, or 907.18474 kg exactly; the British long ton is 2,240 pounds, or 1,016.0469088 kg exactly. The metric tonne is exactly 1,000 kg.

The time factors are equally exact: a minute is 60 seconds and an hour is 3,600 seconds, so pounds per hour is 0.45359237 ÷ 3,600 = 1.259978806 × 10⁻⁴ kg/s, and tonnes per hour is 1,000 ÷ 3,600 = 0.2777778 kg/s. No measured constant enters anywhere, which means every conversion on this page is exact arithmetic to the precision displayed. The kilogram itself is now defined by fixing the Planck constant, as described in the BIPM SI Brochure.

Reference Table: Every Supported Unit

Each factor is the number of kilograms per second in one of that unit.

• kilogram per second (kg/s) — 1 (SI base for this quantity)
• kilogram per minute (kg/min) — 0.0166666667 kg/s
• kilogram per hour (kg/h) — 2.77777778 × 10⁻⁴ kg/s
• gram per second (g/s) — 1 × 10⁻³ kg/s
• gram per minute (g/min) — 1.66666667 × 10⁻⁵ kg/s
• milligram per second (mg/s) — 1 × 10⁻⁶ kg/s
• tonne per hour (t/h) — 0.277777778 kg/s
• tonne per day (t/d) — 0.0115740741 kg/s
• pound per second (lb/s) — 0.45359237 kg/s exactly
• pound per minute (lb/min) — 7.55987283 × 10⁻³ kg/s
• pound per hour (lb/h) — 1.25997881 × 10⁻⁴ kg/s
• ounce per second (oz/s) — 0.028349523125 kg/s exactly
• short ton per hour (US ton/h) — 0.251995761 kg/s
• long ton per hour (UK ton/h) — 0.282235252 kg/s

Three Kinds of Ton, and Why It Matters

Anyone converting bulk material rates runs into the ton problem. The metric tonne is 1,000 kg. The US short ton is 2,000 pounds, about 907.2 kg. The British long ton is 2,240 pounds, about 1,016.0 kg. The spread between the smallest and largest is about twelve percent, which on a plant handling hundreds of tonnes an hour is a very large number of real kilograms.

Spelling offers a hint but not a guarantee: "tonne" almost always means the metric unit, while "ton" is ambiguous and depends entirely on where the document came from. American engineering documents mean short tons unless they say otherwise. Older British documents often mean long tons. International standards use tonnes. This converter lists all three explicitly rather than offering a generic "ton", because a generic entry would force the tool to guess on the reader's behalf. When a source is unclear, the safest move is to look for a cross-check figure elsewhere in the document and see which interpretation makes the numbers consistent.

What Mass Flow Meters Actually Measure

Most flow instruments do not measure mass directly. An orifice plate or a turbine meter responds to volumetric flow or to velocity, and any mass figure it reports has been computed by multiplying through an assumed or measured density. If that density assumption is wrong — because the fluid is hotter than expected, or the gas is at a different pressure, or the composition has drifted — the reported mass flow is wrong even though the instrument is working perfectly.

Coriolis meters are the exception. They measure mass flow directly by detecting the twisting of a vibrating tube as fluid passes through it, an effect that depends on mass in motion rather than volume. That is why Coriolis meters are specified for custody transfer and for any application where density is variable or unknown, despite costing considerably more. Thermal mass flow meters take a different direct route, inferring mass from how much heat a flowing gas carries away, though they depend on the gas composition being known. When you convert a mass flow figure, it is worth knowing which type of instrument produced it, because the number's reliability differs enormously.

Mass Flow in Energy and Combustion Calculations

Mass flow is the quantity that connects a fuel stream to a heat release. Multiply a fuel's mass flow rate by its calorific value in joules per kilogram and you get thermal power in watts. That calculation only works in consistent units, which usually means converting a fuel rate quoted in kilograms per hour into kilograms per second first, then handling the joules with our energy converter and the resulting watts with the power converter.

The same logic governs steam systems, where a boiler rated in kilograms of steam per hour is really being rated on heat output, and cooling systems, where the heat a coolant stream can carry is its mass flow times its specific heat capacity times the temperature rise. That last calculation needs the temperature converter for the temperature difference, alongside a specific heat capacity figure for the fluid. In every one of these, using a volumetric figure where a mass figure belongs introduces an error equal to the density — which is precisely the mistake this page is built to prevent.

Common Mistakes to Avoid

• Treating kg/s and L/min as convertible — they are different quantities and the bridge is density.
• Using a generic "ton" — metric tonne, short ton and long ton differ by up to twelve percent.
• Trusting a standard-volumetric gas figure without checking the reference temperature and pressure it assumes.
• Mixing time bases — per second and per hour differ by 3,600, and the unit strings look similar in a spreadsheet.
• Assuming an inferential meter's mass reading is direct — most compute it from volume and an assumed density.

Related Free Tools From Arb Digital

Use the flow rate converter for volumetric flow in litres and cubic metres, and the density converter to bridge between the two. The weight converter handles static mass, and the power converter and energy converter cover thermal calculations built on a fuel or coolant mass flow. Everything else is listed in the free online tools hub.

Frequently Asked Questions