Cold Water Storage Tank Calculators

ATCM have designed specific water storage technical calculators for you to use to assist you with the design and installation of cold water storage for premises.

There are two main factors to calculate:

a) Air Gaps  and b) Tank Size

You will find further information and links below.

Air Gaps and Design Implications

Prior to 1999, all water supplies were governed by the ‘Water Supply Bylaws Guide’.  It was then superceded by ‘The Water Supply (Water Fittings) Regulations 1999’ which came into force on July 1st, 1999.   The regulations incorporated many much needed changes.

Click on the attached link to gain a better overview of these changes by reading the ‘Keraflo Guide to the Water Supply Regulations’.  This is a downloadable file..

An important aspect of the 1999 Regulation was the consideration of ‘backflow’ prevention.  New backflow prevention requirements became mandatory so the regulations introduced an increase in the number and type of air gaps.

An air gap is the form of backflow prevention device most preferred by the UK Water Authorities.  If a vacuum should occur in the supply pipe, air will be draw in rather than water.  In order to comply with the Water Supply Regulations a minimum vertical distance must be provided between the outlet of the supply pipe and the stored water.  This vertical height is sized on twice the bore of the incoming feed pipe with a minimum of 20mm.”

Are you sure you need Category 5, Type AB Spill Slot / Air Gap protection on your Inlet?

If you do, proceed to the ATCM Type AB Air Gap and Spill Slot Calculator for design details


In the vast majority of commercial installations a simple Type AF Air Gap, providing Category 4 risk protection, is more than sufficient for 99% of all tank and cistern installations.

This is a much simpler and lower cost solution, details of which can be obtained from WRAS publication, IGN No. 9-04-04, Section 11.

Currently many of the specified Type AB Spill Slots are being removed or blocked off on new tank assemblies on direction of the Local Water Inspector at considerable additional cost and disturbance to the contact.

Current Water Regulations require all new or substantially modified water supply installations to be submitted to and approved by the local Water Undertaking before Installation commences.  Make sure you benefit from this service.

ATCM AB Air Gap and Spill Slot Calculator

This calculation is based on the updated European and British Standard, BS EN 13077:2008.

Enter Tank Inlet size, Weir Slot width and % free area of its Screen (if fitted) and the position of the Critical Water Level within the tank is advised and whether the Spill Slot design is Regulation compliant.

Click the button below to begin your calculation.

Click for Calculator
Image of Engineering technical papers

TIP – ATCM AB Air Gap and Spill Slot

Image of Engineering technical papers. Used on Cistern Inlet Calculations page.

ATCM Cistern / Tank Capacity Calculator

This calculation is based on BS 6700:1997 recommendations.

Dependant on the duty or application selected, the “Actual Useable Capacity” is calculated.  Guidance is advised as to the required “Nominal Capacity” in order to determine the physical dimensions of the cistern or tank.

Also advised is the Nominal Capacity of an associated Break or Booster tank should this be a requirement.

Click the button below to begin your calculation.

Click for Calculator

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