Pressure & Head Conversion

Pressure and head are the same statement twice: p = ρgh. Sea water at 1025 kg/m³ puts 10.06 kPa on every metre of depth, fresh water 9.81 kPa — so a metre of water is about a tenth of a bar, and 10 m is about one atmosphere. Everything below follows from that.

kPa	bar	psi	m of sea water	m of fresh water	ft of sea water
1	0.010	0.15	0.10	0.10	0.3
2	0.020	0.29	0.20	0.20	0.7
5	0.050	0.73	0.50	0.51	1.6
10	0.100	1.45	0.99	1.02	3.3
15	0.150	2.18	1.49	1.53	4.9
20	0.200	2.90	1.99	2.04	6.5
25	0.250	3.63	2.49	2.55	8.2
30	0.300	4.35	2.98	3.06	9.8
40	0.400	5.80	3.98	4.08	13.1
50	0.500	7.25	4.97	5.10	16.3
60	0.600	8.70	5.97	6.12	19.6
80	0.800	11.60	7.96	8.16	26.1
100	1.000	14.50	9.95	10.20	32.6
150	1.500	21.76	14.92	15.30	49.0
200	2.000	29.01	19.90	20.39	65.3
300	3.000	43.51	29.85	30.59	97.9
500	5.000	72.52	49.74	50.99	163.2

By depth

Static head only — the pressure on a hull under way, slamming or in a wave crest is a different and much larger question, and it comes from the classification rule, not from ρgh.

Depth in sea water, m	kPa	psi	bar
0.5	5.0	0.73	0.050
1	10.1	1.46	0.101
2	20.1	2.92	0.201
3	30.2	4.37	0.302
5	50.3	7.29	0.503
7.5	75.4	10.93	0.754
10	100.5	14.58	1.005
15	150.8	21.87	1.508
20	201.0	29.16	2.010
30	301.6	43.74	3.016
50	502.6	72.89	5.026

g = 9.80665 m/s². Sea water density varies with temperature and salinity; 1025 kg/m³ is the usual design value and 1000 for fresh. Feet of head are given because tank gauges and pump curves are still sold in them.

Related: plate panel for what that pressure does to plating, and displacement & hydrostatics for what it does to a hull.

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