Concrete mix ratio lookup

Nominal mixes stop at M20. IS 456 Cl. 9 permits volume-proportioned nominal mixes only up to M20, and those proportions are defined for 20 mm aggregate. M25 and above must be design mixes per IS 10262:2019, with trial mixes and cube tests confirming target mean strength (f_ck + 1.65 × standard deviation). Ratios shown for M25 and above are indicative starting points only.

Aggregate size changes water demand. Water content is adjusted from IS 10262:2019 Table 4 (10 mm — 208 kg/m³, 20 mm — 186, 40 mm — 165, at 50 mm slump, angular aggregate, Zone II sand). Cement moves with water to hold w/c constant. The sand-to-coarse split follows Table 5 — bigger stone means less sand. 80 mm sits outside IS 10262; that figure follows ACI 211.1 for 75 mm and applies only to mass concrete, never to reinforced sections.

Match aggregate size to the section. IS 456 Cl. 5.3.3: nominal max size must not exceed one-quarter of the minimum member thickness, and must clear the reinforcement spacing and cover. 10 mm for thin or congested sections, 20 mm for general RCC, 40 mm for mass and lean concrete, 80 mm only for dams and thick unreinforced pours.

Blended cements aren't a like-for-like swap.PPC (IS 1489) and PSC (IS 455) already contain fly ash or slag, so the quantity is unchanged — but early strength arrives slower. Extend curing to 10–14 days versus 7 for OPC, and don't strike formwork on the 7-day cube alone.

Site fly ash or GGBS raises total cementitious content. Per IS 10262:2019, when fly ash replaces part of the OPC, total cementitious content is increased (roughly 10% at 30% replacement) to hold 28-day strength. GGBS needs a smaller bump. The tool applies these, splits the total into OPC and SCM, and reports w/cm — water to cementitious, not water to cement.

Fly ash and GGBS limits. IS 456 Cl. 5.2 permits fly ash conforming to IS 3812 (Part 1) at 15–35% and GGBS per IS 16714 at 25–70%, provided uniform blending is established. Cold-weather pours and early-strength jobs should sit at the low end.

Durability governs more often than strength. IS 456 Table 5 sets minimum grade, maximum w/c and minimum cement content by exposure — these routinely exceed what the structural design alone demands.

Conversions used here. Cement bag = 50 kg, so 1 tonne = 20 bags; loose bag volume ≈ 0.035 m³. Sand at 1550 kg/m³, coarse aggregate at 1600 kg/m³, fly ash at 1100 kg/m³ loose, GGBS at 1200 kg/m³. Confirm against your actual delivered material — bulk density shifts with grading and moisture.

Dry volume ≈ 1.52 × wet volume. Already applied to the sand and aggregate figures shown. Aggregate totals are held constant across binder changes rather than re-solved by absolute volume — close enough for procurement, not a substitute for a designed mix.

The w/c shown is the specified ceiling, not the back-calculated ratio. It comes from IS 456 Table 5 as a durability limit. The cement content in these tables is generous against it — M20 at 8 bags and 180 L actually works out near 0.45, comfortably under the 0.50 permitted. Treat the stated figure as the number you must not exceed on site.

Water is the number to watch. Litres shown are total mixing water at SSD condition. Deduct free surface moisture in the sand, add for absorption by dry aggregate, or your effective w/c drifts above what was specified.

Workability (IS 456 Cl. 7 / IS 1199). 25–50 mm slump for mass concrete and dry foundations; 50–75 mm for normal slabs and beams; 75–100 mm for heavily reinforced and pumped sections; 100–150 mm for high-rise pumping. Above 150 mm without a superplasticizer, expect segregation.