Mass Flow Converter (kg/s, kg/h, t/h, lb/h)

Understanding the mass flow converter — concept and principle

Mass flow rate is the mass of material passing a point per unit of time. It is the natural way to describe steam from a boiler, fuel to a burner or engine, feed into a crusher or conveyor, and raw materials in chemical and food plants, because mass is conserved even when temperature and pressure change the volume. This converter switches between kilograms per second (kg/s), kilograms per hour (kg/h), metric tonnes per hour (t/h), pounds per hour (lb/h) and pounds per second (lb/s).

Convert the mass and the time separately: tonnes → kg ×1,000, per hour → per second ÷3,600.

How it is calculated

Every unit is converted to kg/h and then to the target: result = value × (kg/h per "from" unit) ÷ (kg/h per "to" unit). The factors are:

1 kg/s = 3,600 kg/h (an hour has 3,600 seconds)
1 t/h = 1,000 kg/h
1 lb/h = 0.45359237 kg/h (the international pound is defined as exactly 0.45359237 kg)
1 lb/s = 0.45359237 × 3,600 = 1,632.932532 kg/h

The principle

A flow rate is a quantity divided by a time, so converting it means converting the mass unit and the time unit separately and combining them. Going from tonnes to kilograms multiplies by 1,000; going from "per hour" to "per second" divides by 3,600. Mass flow is linked to volumetric flow by density: ṁ = ρ × Q, where ρ is density in kg/m³ and Q is volume flow in m³/h or m³/s. Engineers prefer mass flow for gases and steam because their density changes strongly with pressure and temperature, while mass is conserved through every pipe, valve and heat exchanger.

Worked example

Inputs: value = 1, from t/h, to kg/h.
1 t/h × 1,000 ÷ 1 = 1,000 kg/h.
In kg/s: 1,000 ÷ 3,600 = 0.2777777778 kg/s.
In lb/h: 1,000 ÷ 0.45359237 = 2,204.622622 lb/h; in lb/s: 2,204.622622 ÷ 3,600 = 0.6123951727 lb/s.

How to read the result

Quick rules: kg/s × 3.6 = t/h, and t/h ÷ 3.6 = kg/s. Boiler capacities are often quoted in t/h of steam, process lines in kg/h and engine fuel or air flow in kg/s or lb/s. When comparing equipment from US and metric suppliers, convert both to one unit before comparing.

Common mistakes

Confusing metric tonnes (1,000 kg) with US short tons (907.18474 kg) or long tons (1,016.0469088 kg).
Treating a volume flow (m³/h, L/min) as a mass flow without multiplying by the fluid's density.
Mixing up per hour and per second, which gives an error of 3,600 times.

Keep in mind: Converting between mass and volume flow requires the density at actual operating conditions. For gases and steam, use the density at the real pressure and temperature, not a table value at standard conditions.

Frequently asked questions

How do I convert kg/s to t/h?

Multiply by 3.6. For example, 2 kg/s = 7.2 t/h.

How many lb/h is 1 kg/h?

About 2.204623 lb/h.

How do I get mass flow from volume flow?

Multiply by density. Water at room temperature has roughly 1,000 kg/m³, so 1 m³/h of water is about 1,000 kg/h.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition, 2019 · NIST Special Publication 811 (2008), Appendix B

Checked 8 Nov 2026. Factors from the SI Brochure (BIPM, 9th ed.) and NIST Special Publication 811 (2008), Appendix B.