Concrete Slump Calculator

How the concrete slump calculator works

Enter a target slump, in inches on the US units side or millimetres on the metric side. The calculator does not change the value: it converts it to the other unit, places it in the European EN 206 consistence classes, lists the types of construction whose ACI 211.1 slump range includes it, gives the range of test results that ASTM C94 accepts for that target, and flags values outside the range where the slump test is meaningful. The formula block shows the conversion, 1 in = 25.4 mm.

What slump measures

The slump test measures how far a cone of fresh concrete sags once the mould is lifted away. ASTM C143 covers the test in the laboratory and in the field, and the European equivalent is EN 12350-2. It is a check on consistency, not strength. ASTM notes that under laboratory control slump rises with water content, and so tends to go with lower strength, but that on site the link is not consistent, so a slump result should not be read as a strength result. The test applies to plastic concrete with coarse aggregate up to 1½ in (37.5 mm).

ASTM also notes that concrete with less than ½ in (15 mm) of slump may not be plastic enough, and concrete with more than about 9 in (230 mm) may not be cohesive enough, for the test to mean much. EN 12350-2 describes the test as sensitive between 10 and 210 mm, and unsuitable when the largest aggregate is over 40 mm or when the slump keeps changing for more than a minute after the cone is lifted. The result flags values outside these ranges, using the ASTM limits on the US units side and both sets on the metric side. Very fluid, self-consolidating concrete is tested for slump flow under ASTM C1611 instead.

EN 206 consistence classes

In Europe, the UK and Ireland, concrete is ordered by consistence class. Roadstone’s guide to I.S. EN 206 lists five slump classes: S1 for 10 to 40 mm, S2 for 50 to 90 mm, S3 for 100 to 150 mm, S4 for 160 to 210 mm, and S5 for 220 mm and over. A 75 mm slump, once a common figure for foundations, is an S2. Values that fall in the gaps, such as 45 mm, belong to no class, and the result says which two classes they sit between.

Typical slumps by type of construction

ACI 211.1, the American mix-proportioning practice, gives general slump ranges by type of construction and recommends the stiffest mix that can still be placed adequately. As reproduced by Pavement Interactive, a reference built by several US state highway departments, the FHWA and the University of Washington, they are 1 to 3 in (25 to 75 mm) for footings, foundation walls, caissons, pavements and slabs; 1 to 4 in (25 to 100 mm) for beams, reinforced walls and building columns; and 1 to 2 in (25 to 50 mm) for mass concrete. The same page lists typical state specifications for slip-form paving of 0 to 3 in. The result names every group whose range includes your target, so 3 in (76 mm) covers footings, slabs, beams, walls and columns but not mass concrete.

Acceptable test results

A slump test on a delivered load never lands exactly on the target, so specifications allow a tolerance. NRMCA’s guide to acceptance testing summarises the ASTM C94 figures for a slump specified as a target: ±½ in (15 mm) for targets up to 2 in (50 mm), ±1 in (25 mm) for targets over 2 in and up to 4 in (100 mm), and ±1½ in (40 mm) above 4 in. A 3 in target therefore accepts results from 2 to 4 in; a 75 mm target accepts 50 to 100 mm; a 2 in target accepts 1½ to 2½ in. Where the slump is specified as a maximum, ASTM C94 uses different limits, +0 to −1½ in (40 mm) up to 3 in (75 mm) and +0 to −2½ in (65 mm) above, which this tool does not apply.

ASTM gives its inch and millimetre values as separate systems rather than exact conversions: ½ in is 12.7 mm, but the metric tolerance is 15 mm. The calculator uses the inch figures on the US side and the millimetre figures on the metric side. Irish practice under I.S. EN 206 is different again: Roadstone's guide lists ±30 mm for targets up to 40 mm, ±40 mm for 50 to 90 mm and ±50 mm from 100 mm, so a 75 mm target there accepts 35 to 115 mm. Your project specification sets which rule applies.

On site

NRMCA describes a preliminary sample taken after about 0.25 yd³ (0.20 m³) has been discharged, used to check slump and air content and adjust the load if needed. If that preliminary sample comes in below the specified slump, ASTM C94 allows the load to be adjusted with water or admixtures and then mixed thoroughly. If a test fails the specification, a retest on a separate sample is made before the load is rejected. Adding water raises slump but, as ASTM's note on water content suggests, it can also lower strength, so any water added on site should stay within what the mix design allows.

What this tool does not do

It does not design a mix, predict strength, or tell you what slump your job needs. Those come from the project specification, the engineer or the concrete supplier. It classifies a slump value you already have and shows how the main standards treat it.

Using this calculator with other BuildMetricLab tools

The concrete slab calculator and concrete footing calculator work out the volume to order, the concrete mix ratio calculator covers site-mixed proportions, and the ready-mix truck calculator helps plan deliveries. All BuildMetricLab tools run in your browser with no sign-up, and every formula is shown on the page so the maths can be checked.