CM postcode district map, Chelmsford
Supplier statuses last checked .
The CM postcode area covers 28 districts and 18,674 live postcodes, from CM0 to CM99. It is centred on Chelmsford and also covers Harlow, Brentwood, Braintree, South Woodham Ferrers and Billericay.
Districts here average 667 postcodes.
Where the CM districts sit
7 districts are plotted without a label where labels would overlap; every one is in the table below.
Where CM sits
It falls across Essex and Hertfordshire.
Water supply across CM
All 28 CM districts are supplied by Anglian Water, which is notable at this size: an area with this many districts usually crosses at least one supply boundary, and this one does not.
Anglian Water has a hosepipe ban in force from 11 July 2026, covering the whole of CM.
The shape of the CM area
CM3 is the largest district with 1,588 postcodes around South Woodham Ferrers, and CM92 the smallest with 18. The area divides into 99 postcode sectors.
The numbering is not continuous. CM runs up to 99 but has only 28 districts, so 72 numbers are unused:.
Several districts share a settlement, which is a common source of confusion when a form rejects an address: CM17, CM18, CM19, CM20 and CM92 all point at Harlow, CM1, CM2, CM98 and CM99 all point at Chelmsford and CM4, CM11 and CM12 all point at Billericay.
What borders CM
CM sits next to SS (Southend-on-Sea) to the south east, RM (Chigwell) to the south west, IG (Chigwell) to the south west, EN (Waltham Cross) to the west and E (City of London) to the south west.
CM districts, settlements and water supply
Principal settlement is the town or city each district sits in or nearest to, derived from OS Open Names. It is not the Royal Mail post town, which is licensed data and not published openly. Status is resolved live from each company’s own notice.
Water hardness in CM
No water company covering CM publishes hardness figures as open data. The gap is named per district below so you know who to ask.
| District | Hardness | As published | Supply zone | Published by |
|---|---|---|---|---|
| CM0 | Not published by Anglian Water. Check with your supplier. | |||
| CM1 | Not published by Anglian Water. Check with your supplier. | |||
| CM2 | Not published by Anglian Water. Check with your supplier. | |||
| CM3 | Not published by Anglian Water. Check with your supplier. | |||
| CM4 | Not published by Anglian Water. Check with your supplier. | |||
| CM5 | Not published by Anglian Water. Check with your supplier. | |||
| CM6 | Not published by Anglian Water. Check with your supplier. | |||
| CM7 | Not published by Anglian Water. Check with your supplier. | |||
| CM8 | Not published by Anglian Water. Check with your supplier. | |||
| CM9 | Not published by Anglian Water. Check with your supplier. | |||
| CM11 | Not published by Anglian Water. Check with your supplier. | |||
| CM12 | Not published by Anglian Water. Check with your supplier. | |||
| CM13 | Not published by Anglian Water. Check with your supplier. | |||
| CM14 | Not published by Anglian Water. Check with your supplier. | |||
| CM15 | Not published by Anglian Water. Check with your supplier. | |||
| CM16 | Not published by Anglian Water. Check with your supplier. | |||
| CM17 | Not published by Anglian Water. Check with your supplier. | |||
| CM18 | Not published by Anglian Water. Check with your supplier. | |||
| CM19 | Not published by Anglian Water. Check with your supplier. | |||
| CM20 | Not published by Anglian Water. Check with your supplier. | |||
| CM21 | Not published by Anglian Water. Check with your supplier. | |||
| CM22 | Not published by Anglian Water. Check with your supplier. | |||
| CM23 | Not published by Anglian Water. Check with your supplier. | |||
| CM24 | Not published by Anglian Water. Check with your supplier. | |||
| CM77 | Not published by Anglian Water. Check with your supplier. | |||
| CM92 | Not published by Anglian Water. Check with your supplier. | |||
| CM98 | Not published by Anglian Water. Check with your supplier. | |||
| CM99 | Not published by Anglian Water. Check with your supplier. | |||
Hardness is published per census area, not per postcode, so these figures characterise a district rather than measuring one address. They are a scale and taste matter, not a safety one. Hardness data version wessex-water@2022-2025+yorkshire-water@2026.
Download the CM data
No signup, no email wall. GeoJSON carries the district points, the supplier and the live status, so it can be dropped straight into a map.
District positions are averaged postcode centroids from OS Code-Point Open. They show roughly where a district sits, not where its boundary runs, and are not legal boundaries.
Frequently asked questions
How many postcode districts are in the CM area?
There are 28 CM postcode districts, running from CM0 to CM99, and they contain 18,674 live postcodes across 99 sectors.
What towns does the CM postcode area cover?
CM covers Chelmsford, Harlow, Brentwood, Braintree, South Woodham Ferrers and Billericay. The area takes its name from Chelmsford, and each district is listed with its settlement in the table on this page.
Which water company supplies CM postcodes?
Anglian Water supplies all 28 CM districts. Supply boundaries follow the pipe network rather than postcode geography, so confirm against your water bill if you are near the edge of the area.
Why do CM17 and CM18 both say Harlow?
CM17, CM18, CM19, CM20 and CM92 all serve Harlow, covering different parts of it. A single town routinely needs more than one postcode district once it passes a few thousand delivery points, so this is normal rather than an error in an address.
Can I download a free CM postcode map?
Yes. This page offers the CM districts as GeoJSON, CSV and SVG, free and with no signup or email address required. The GeoJSON carries each district's position, its water company and its current restriction status, so it can be dropped straight into a mapping tool.
Why is the CM area called Chelmsford when it covers South Woodham Ferrers?
CM takes its letters from Chelmsford, where the sorting office that served the area was based when the code was allocated. South Woodham Ferrers is the largest settlement in the area today, in CM3, but the area code follows the historic sorting geography rather than current population.
Which councils cover the CM postcode area?
CM reaches into Essex and Hertfordshire. A postcode area is not a council area: they were drawn for different purposes and a single district can sit across a council boundary, so your postcode does not tell you which authority you pay.