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Water engineering unit converter

Convert between the units used in UK water and wastewater engineering: flow in L/s, m³/h, ML/d and MGD, pressure and head in bar, kPa, metres of water and psi, concentration and load in mg/L, kg/d and meq/L, surface loading in m/h and gpm/ft², area, volume, power, specific energy and temperature. Pick a category and the converter shows your value in every unit of that category at once. Below the converter are complete one-unit tables for each category and an ion table with equivalent weights, so the page is a reference as well as a tool.

Converter

Result

Converted value60 L/min1 L/s

Factor601 L/s = 60 L/min

UK and US gallons differ by 20%: 1 MGD (UK) is 4,546 m³/d and 1 MGD (US) is 3,785 m³/d. Say which you mean.
1 ML/d = 11.57 L/s = 41.67 m³/h. A works of 10 ML/d DWF serves roughly 50,000 to 70,000 people.

1 L/s in every unit

Unit	Value	Factor to L/s
L/s	1	1
L/min	60	0.01667
m³/h	3.6	0.2778
m³/d	86.4	0.01157
m³/s	0.001	1,000
ML/d (megalitres a day)	0.0864	11.57
MGD (UK, million imperial gallons a day)	0.01901	52.62
MGD (US, million US gallons a day)	0.02282	43.81
gpm (US)	15.85	0.06309
gpm (UK)	13.2	0.07577
ft³/s (cusec)	0.03531	28.32

Load from concentration and flow

Load250 kg/d551.2 lb/d, 91.25 t/year, 10.42 kg/h

Flow1,000 m³/d11.57 L/s, 1 ML/d

Ions: mg/L to meq/L, mmol/L and mg/L as CaCO₃

Equivalents4.99 meq/Lequivalent weight 20.04 g/eq (40.08 ÷ 2)

As CaCO₃249.7 mg/Lmeq/L × 50.043; 2.495 mmol/L

Ion	Molar mass, g/mol	Charge	Equivalent weight, g/eq	1 mg/L = meq/L	1 mg/L = mg/L as CaCO₃
Calcium, Ca²⁺	40.078	2	20.04	0.0499	2.497
Magnesium, Mg²⁺	24.305	2	12.15	0.08229	4.118
Sodium, Na⁺	22.99	1	22.99	0.0435	2.177
Potassium, K⁺	39.098	1	39.1	0.02558	1.28
Ammonium, NH₄⁺	18.039	1	18.04	0.05544	2.774
Iron(II), Fe²⁺	55.845	2	27.92	0.03581	1.792
Iron(III), Fe³⁺	55.845	3	18.62	0.05372	2.688
Aluminium, Al³⁺	26.982	3	8.994	0.1112	5.564
Manganese, Mn²⁺	54.938	2	27.47	0.0364	1.822
Bicarbonate, HCO₃⁻	61.017	1	61.02	0.01639	0.8201
Carbonate, CO₃²⁻	60.009	2	30	0.03333	1.668
Hydroxide, OH⁻	17.007	1	17.01	0.0588	2.942
Chloride, Cl⁻	35.453	1	35.45	0.02821	1.412
Sulphate, SO₄²⁻	96.06	2	48.03	0.02082	1.042
Nitrate, NO₃⁻	62.005	1	62.01	0.01613	0.8071
Phosphate, PO₄³⁻	94.971	3	31.66	0.03159	1.581
Fluoride, F⁻	18.998	1	19	0.05264	2.634

Conversion tables: one unit in every other unit

Each row reads "1 of the row unit equals the values across". Factors are exact where the definitions are exact (gallons, feet, pounds, atmospheres) and rounded to four significant figures for display.

Flow

1 unit =	L/s	L/min	m³/h	m³/d	m³/s	ML/d (megalitres a day)	MGD (UK, million imperial gallons a day)	MGD (US, million US gallons a day)	gpm (US)	gpm (UK)	ft³/s (cusec)
1 L/s	1	60	3.6	86.4	0.001	0.0864	0.01901	0.02282	15.85	13.2	0.03531
1 L/min	0.01667	1	0.06	1.44	0.00001667	0.00144	0.0003168	0.0003804	0.2642	0.22	0.0005886
1 m³/h	0.2778	16.67	1	24	0.0002778	0.024	0.005279	0.00634	4.403	3.666	0.00981
1 m³/d	0.01157	0.6944	0.04167	1	0.00001157	0.001	0.00022	0.0002642	0.1835	0.1528	0.0004087
1 m³/s	1,000	60,000	3,600	86,400	1	86.4	19.01	22.82	15,850	13,198	35.31
1 ML/d (megalitres a day)	11.57	694.4	41.67	1,000	0.01157	1	0.22	0.2642	183.5	152.8	0.4087
1 MGD (UK, million imperial gallons a day)	52.62	3,157	189.4	4,546	0.05262	4.546	1	1.201	834	694.4	1.858
1 MGD (US, million US gallons a day)	43.81	2,629	157.7	3,785	0.04381	3.785	0.8327	1	694.4	578.2	1.547
1 gpm (US)	0.06309	3.785	0.2271	5.451	0.00006309	0.005451	0.001199	0.00144	1	0.8327	0.002228
1 gpm (UK)	0.07577	4.546	0.2728	6.546	0.00007577	0.006546	0.00144	0.001729	1.201	1	0.002676
1 ft³/s (cusec)	28.32	1,699	101.9	2,447	0.02832	2.447	0.5382	0.6463	448.8	373.7	1

Pressure and head

1 unit =	kPa	Pa	bar	mbar	m H₂O (conventional)	mm H₂O	ft H₂O	psi (lbf/in²)	mm Hg (Torr)	in Hg	atm	kgf/cm²
1 kPa	1	1,000	0.01	10	0.102	102	0.3346	0.145	7.501	0.2953	0.009869	0.0102
1 Pa	0.001	1	0.00001	0.01	0.000102	0.102	0.0003346	0.000145	0.007501	0.0002953	0.000009869	0.0000102
1 bar	100	100,000	1	1,000	10.2	10,197	33.46	14.5	750.1	29.53	0.9869	1.02
1 mbar	0.1	100	0.001	1	0.0102	10.2	0.03346	0.0145	0.7501	0.02953	0.0009869	0.00102
1 m H₂O (conventional)	9.807	9,807	0.09807	98.07	1	1,000	3.281	1.422	73.56	2.896	0.09678	0.1
1 mm H₂O	0.009807	9.807	0.00009807	0.09807	0.001	1	0.003281	0.001422	0.07356	0.002896	0.00009678	0.0001
1 ft H₂O	2.989	2,989	0.02989	29.89	0.3048	304.8	1	0.4335	22.42	0.8827	0.0295	0.03048
1 psi (lbf/in²)	6.895	6,895	0.06895	68.95	0.7031	703.1	2.307	1	51.71	2.036	0.06805	0.07031
1 mm Hg (Torr)	0.1333	133.3	0.001333	1.333	0.0136	13.6	0.0446	0.01934	1	0.03937	0.001316	0.00136
1 in Hg	3.386	3,386	0.03386	33.86	0.3453	345.3	1.133	0.4912	25.4	1	0.03342	0.03453
1 atm	101.3	101,325	1.013	1,013	10.33	10,332	33.9	14.7	760	29.92	1	1.033
1 kgf/cm²	98.07	98,067	0.9807	980.7	10	10,000	32.81	14.22	735.6	28.96	0.9678	1

Concentration

1 unit =	mg/L	g/m³	ppm (by mass, dilute water)	µg/L	ppb	g/L	kg/m³	% w/v	lb per million US gallons	grains per US gallon
1 mg/L	1	1	1	1,000	1,000	0.001	0.001	0.0001	8.345	58.42
1 g/m³	1	1	1	1,000	1,000	0.001	0.001	0.0001	8.345	58.42
1 ppm (by mass, dilute water)	1	1	1	1,000	1,000	0.001	0.001	0.0001	8.345	58.42
1 µg/L	0.001	0.001	0.001	1	1	0.000001	0.000001	1.0e-7	0.008345	0.05842
1 ppb	0.001	0.001	0.001	1	1	0.000001	0.000001	1.0e-7	0.008345	0.05842
1 g/L	1,000	1,000	1,000	1,000,000	1,000,000	1	1	0.1	8,345	58,418
1 kg/m³	1,000	1,000	1,000	1,000,000	1,000,000	1	1	0.1	8,345	58,418
1 % w/v	10,000	10,000	10,000	10,000,000	10,000,000	10	10	1	83,454	584,178
1 lb per million US gallons	0.1198	0.1198	0.1198	119.8	119.8	0.0001198	0.0001198	0.00001198	1	7
1 grains per US gallon	0.01712	0.01712	0.01712	17.12	17.12	0.00001712	0.00001712	0.000001712	0.1429	1

Velocity and surface loading

1 unit =	m/h (= m³/m²/h)	m/s	m/d (= m³/m²/d)	mm/h	L/m²/h	L/m²/s	ft/s	gpm/ft² (US)	gpd/ft² (US)
1 m/h (= m³/m²/h)	1	0.0002778	24	1,000	1,000	0.2778	0.0009113	0.409	589
1 m/s	3,600	1	86,400	3,600,000	3,600,000	1,000	3.281	1,473	2,120,462
1 m/d (= m³/m²/d)	0.04167	0.00001157	1	41.67	41.67	0.01157	0.00003797	0.01704	24.54
1 mm/h	0.001	2.778e-7	0.024	1	1	0.0002778	9.113e-7	0.000409	0.589
1 L/m²/h	0.001	2.778e-7	0.024	1	1	0.0002778	9.113e-7	0.000409	0.589
1 L/m²/s	3.6	0.001	86.4	3,600	3,600	1	0.003281	1.473	2,120
1 ft/s	1,097	0.3048	26,335	1,097,280	1,097,280	304.8	1	448.8	646,317
1 gpm/ft² (US)	2.445	0.0006791	58.67	2,445	2,445	0.6791	0.002228	1	1,440
1 gpd/ft² (US)	0.001698	4.716e-7	0.04075	1.698	1.698	0.0004716	0.000001547	0.0006944	1

Area

1 unit =	m²	hectare	km²	ft²	yd²	acre	mile²
1 m²	1	0.0001	0.000001	10.76	1.196	0.0002471	3.861e-7
1 hectare	10,000	1	0.01	107,639	11,960	2.471	0.003861
1 km²	1,000,000	100	1	10,763,910	1,195,990	247.1	0.3861
1 ft²	0.0929	0.00000929	9.29e-8	1	0.1111	0.00002296	3.587e-8
1 yd²	0.8361	0.00008361	8.361e-7	9	1	0.0002066	3.228e-7
1 acre	4,047	0.4047	0.004047	43,560	4,840	1	0.001563
1 mile²	2,589,988	259	2.59	27,878,400	3,097,600	640	1

Volume

1 unit =	m³	litre	ML (megalitre)	gallon (UK)	gallon (US)	million gallons (UK)	million gallons (US)	ft³	acre-foot	barrel (oil, 42 US gal)
1 m³	1	1,000	0.001	220	264.2	0.00022	0.0002642	35.31	0.0008107	6.29
1 litre	0.001	1	0.000001	0.22	0.2642	2.2e-7	2.642e-7	0.03531	8.107e-7	0.00629
1 ML (megalitre)	1,000	1,000,000	1	219,969	264,172	0.22	0.2642	35,315	0.8107	6,290
1 gallon (UK)	0.004546	4.546	0.000004546	1	1.201	0.000001	0.000001201	0.1605	0.000003686	0.02859
1 gallon (US)	0.003785	3.785	0.000003785	0.8327	1	8.327e-7	0.000001	0.1337	0.000003069	0.02381
1 million gallons (UK)	4,546	4,546,090	4.546	1,000,000	1,200,950	1	1.201	160,544	3.686	28,594
1 million gallons (US)	3,785	3,785,412	3.785	832,674	1,000,000	0.8327	1	133,681	3.069	23,810
1 ft³	0.02832	28.32	0.00002832	6.229	7.481	0.000006229	0.000007481	1	0.00002296	0.1781
1 acre-foot	1,233	1,233,482	1.233	271,328	325,851	0.2713	0.3259	43,560	1	7,758
1 barrel (oil, 42 US gal)	0.159	159	0.000159	34.97	42	0.00003497	0.000042	5.615	0.0001289	1

Power

1 unit =	kW	W	MW	hp (mechanical, 550 ft·lbf/s)	hp (metric, PS)	BTU/h
1 kW	1	1,000	0.001	1.341	1.36	3,412
1 W	0.001	1	0.000001	0.001341	0.00136	3.412
1 MW	1,000	1,000,000	1	1,341	1,360	3,412,142
1 hp (mechanical, 550 ft·lbf/s)	0.7457	745.7	0.0007457	1	1.014	2,544
1 hp (metric, PS)	0.7355	735.5	0.0007355	0.9863	1	2,510
1 BTU/h	0.0002931	0.2931	2.931e-7	0.000393	0.0003985	1

Specific energy (energy per volume treated or pumped)

1 unit =	kWh/m³	Wh/m³	kWh/ML	MJ/m³	kJ/m³	kWh per 1000 US gallons	kWh per million UK gallons
1 kWh/m³	1	1,000	1,000	3.6	3,600	3.785	4,546
1 Wh/m³	0.001	1	1	0.0036	3.6	0.003785	4.546
1 kWh/ML	0.001	1	1	0.0036	3.6	0.003785	4.546
1 MJ/m³	0.2778	277.8	277.8	1	1,000	1.052	1,263
1 kJ/m³	0.0002778	0.2778	0.2778	0.001	1	0.001052	1.263
1 kWh per 1000 US gallons	0.2642	264.2	264.2	0.951	951	1	1,201
1 kWh per million UK gallons	0.00022	0.22	0.22	0.0007919	0.7919	0.0008327	1

Temperature

°C	°F	K	Note
-273.15	-459.7	0	absolute zero
0	32	273.2	ice point
4	39.2	277.2	maximum density of water
10	50	283.2	typical UK winter sewage
15	59	288.2	typical UK groundwater
20	68	293.2	standard conditions for oxygen transfer
25	77	298.2	laboratory reference
35	95	308.2	mesophilic digestion
55	131	328.2	thermophilic digestion
100	212	373.2	boiling point at 1 atm

How the conversion works

Linear units. Each unit has a factor f to the category's base unit (L/s, kPa, mg/L, m/h, m², m³, kW, kWh/m³). Value_to = value_from × f_from ÷ f_to [NIST].
Definitions used. 1 in = 25.4 mm, 1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 lbf = 4.4482216152605 N, 1 US gal = 231 in³ = 3.785411784 L, 1 UK gal = 4.54609 L, 1 atm = 101 325 Pa, 1 mm Hg = 101 325 ÷ 760 Pa, 1 m H₂O (conventional) = 9 806.65 Pa, 1 hp = 745.6999 W, 1 kWh = 3.6 MJ, 1 acre = 4 046.8564224 m² [NIST] [BIPM].
Temperature. °C = (°F − 32) × 5 ÷ 9; K = °C + 273.15 [BIPM].
Load. kg/d = mg/L × m³/d ÷ 1000, since 1 mg/L = 1 g/m³. Flow is first converted to m³/d (L/s × 86.4).
Ions. meq/L = mg/L ÷ (molar mass ÷ charge); mmol/L = mg/L ÷ molar mass; mg/L as CaCO₃ = meq/L × 50.043, the equivalent weight of CaCO₃ (100.087 ÷ 2) [IUPAC].

Worked example with the default inputs

1 L/s to L/min: f_L/s = 1 and f_L/min = 1 ÷ 60, so 1 × 1 ÷ (1 ÷ 60) = 60 L/min. The same litre a second is 3.6 m³/h, 86.4 m³/d, 0.0864 ML/d, 0.019 MGD (UK), 15.85 US gpm and 0.0353 ft³/s. For the load helper, 250 mg/L at 1,000 m³/d is 250 × 1,000 ÷ 1,000 = 250 kg/d. For calcium at 100 mg/L the equivalent weight is 40.078 ÷ 2 = 20.04 g/eq, so 100 ÷ 20.04 = 4.99 meq/L and 4.99 × 50.043 = 250 mg/L as CaCO₃.

What this cannot tell you. The converter is exact for defined units but not for quantities that depend on fluid properties. ppm by mass equals mg/L only at a density of 1 kg/L; metres of water assume 1000 kg/m³; gas flows need a stated temperature and pressure (Nm³/h, Sm³/h) and are not covered here; and "as N", "as P" or "as CaCO₃" expressions require the species to be identified before converting. Significant figures are kept to four for display, which is more than most field measurements justify.

Sources

Frequently asked questions

How many litres per second is 1 ML/d?

1 megalitre a day is 1,000 m³/d, which is 1,000,000 L ÷ 86,400 s = 11.574 L/s, or 41.67 m³/h. Conversely 1 L/s is 86.4 m³/d or 0.0864 ML/d.

What is the difference between UK and US MGD?

A UK (imperial) gallon is 4.54609 L and a US gallon is 3.785411784 L, so 1 MGD (UK) = 4,546 m³/d = 52.6 L/s and 1 MGD (US) = 3,785 m³/d = 43.8 L/s. The 20% gap is large enough to size a pump wrong, so UK practice has moved to ML/d and L/s.

How do I convert mg/L to kg/d?

Multiply the concentration by the daily flow in m³/d and divide by 1,000, because 1 mg/L = 1 g/m³. For example 250 mg/L BOD at 1,000 m³/d is 250 × 1,000 ÷ 1,000 = 250 kg/d. For a flow in L/s, multiply by 86.4 first to get m³/d.

What is mg/L as CaCO3 and how does it relate to meq/L?

Hardness and alkalinity are often expressed as if the ions were calcium carbonate, which has an equivalent weight of 50.04 g/eq. Convert the ion to milliequivalents (mg/L ÷ equivalent weight, where equivalent weight = molar mass ÷ charge) and multiply by 50.04. For calcium: 100 mg/L ÷ 20.04 = 4.99 meq/L = 250 mg/L as CaCO3.

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