MixDesignCalc 2026 — Complete Guide to FA:CA Ratio Selection per IS 10262:2019 Annex A. Interactive FA% Calculator, Two-CA Size Blending Tool, Fineness Modulus Adjustment, Zone Influence, W/C Effect & Absolute Volume Working.
Calculate Aggregate RatiosAggregate proportioning — the selection of the ratio of fine aggregate (sand) to coarse aggregate (gravel or crushed stone) in a concrete mix — is perhaps the most under-appreciated step in mix design. Many engineers simply adopt a fixed ratio (often 40:60 FA:CA) for all mixes without understanding that the optimal split depends critically on: (1) the fineness modulus and grading zone of the fine aggregate; (2) the nominal maximum size of the coarse aggregate; (3) the free water-cement ratio; and (4) the target workability. Using the wrong FA:CA ratio can increase water demand by 10–25 kg/m³, raise cement content by 20–50 kg/m³, increase shrinkage, reduce long-term durability — all without changing the specified grade.
| Ingredient | Mass (kg/m³) | SG | Volume (m³) | % Total |
|---|
The following table presents the complete IS 10262:2019 Annex A Table A-1 recommendations for fine aggregate percentage of total aggregate volume. The FA% range gives a lower and upper bound; the single value to use in design is the middle of the range or the end of the range nearest to Zone II behaviour. Where FM deviates from the zone midpoint, apply the FM correction rule below the table.
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| Max Agg Size | FA Zone | W/C 0.35 | W/C 0.40 | W/C 0.45 | W/C 0.50 | W/C 0.55 | W/C 0.60 | W/C 0.65+ | Notes |
|---|---|---|---|---|---|---|---|---|---|
| 10mm | Zone I | 38–42 | 40–44 | 42–46 | 44–48 | 46–50 | 48–52 | 50–54 | Coarse sand; needs more FA |
| 10mm | Zone II | 34–38 | 36–40 | 38–42 | 40–44 | 42–46 | 44–48 | 46–50 | Standard; 10mm agg |
| 10mm | Zone III | 30–34 | 32–36 | 34–38 | 36–40 | 38–42 | 40–44 | 42–46 | Finer sand; less FA |
| 10mm | Zone IV | 26–30 | 28–32 | 30–34 | 32–36 | 34–38 | 36–40 | 38–42 | Very fine sand; minimum FA |
| 20mm | Zone I | 34–38 | 36–40 | 38–42 | 40–44 | 42–46 | 44–48 | 46–50 | Coarse sand; common in some regions |
| 20mm | Zone II Preferred | 30–34 | 32–36 | 34–38 | 36–40 | 38–42 | 40–44 | 42–46 | Most common: M25–M50 standard |
| 20mm | Zone III | 26–30 | 28–32 | 30–34 | 32–36 | 34–38 | 36–40 | 38–42 | Medium fine; reduce FA |
| 20mm | Zone IV | 22–26 | 24–28 | 26–30 | 28–32 | 30–34 | 32–36 | 34–38 | Very fine sand; lowest FA; caution |
| 40mm | Zone I | 30–34 | 32–36 | 34–38 | 36–40 | 38–42 | 40–44 | 42–46 | Mass concrete; coarse sand |
| 40mm | Zone II | 26–30 | 28–32 | 30–34 | 32–36 | 34–38 | 36–40 | 38–42 | Mass concrete; standard |
| 40mm | Zone III | 22–26 | 24–28 | 26–30 | 28–32 | 30–34 | 32–36 | 34–38 | Mass concrete; finer sand |
| 40mm | Zone IV | 18–22 | 20–24 | 22–26 | 24–28 | 26–30 | 28–32 | 30–34 | Mass concrete; very fine; lowest FA |
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| Parameter | Change | Effect on FA% | Reason | Magnitude |
|---|---|---|---|---|
| FA Grading Zone | Zone II → Zone I (coarser) | Increase FA% by ~4% | Coarser sand packs poorly; needs more FA to fill voids | ±4% per zone change |
| FA Grading Zone | Zone II → Zone III (finer) | Decrease FA% by ~4% | Finer sand has higher surface area; fewer voids; less needed | ±4% per zone change |
| Fineness Modulus | FM increases by 0.1 | Increase FA% by 1.5% | Higher FM = coarser sand = more FA to fill inter-particle voids | 1.5% per 0.1 FM |
| Max Agg Size | 10mm → 20mm | Decrease FA% by ~4% | Larger CA has lower surface area per unit mass; less paste/FA needed | ~4% per size step |
| Max Agg Size | 20mm → 40mm | Decrease FA% by ~4% | Same reason; larger CA allows more efficient packing with less FA | ~4% per size step |
| W/C Ratio | Decrease by 0.10 | Decrease FA% by ~2% | Lower w/c = denser paste; less space for FA; coarser mix acceptable | ~2% per 0.10 w/c |
| M-Sand vs River Sand | Switch to M-Sand | Decrease FA% by ~2–3% | M-sand is more angular; higher void ratio; but also higher surface area — net effect: slight FA reduction | 2–3% |
| Pump Mix | Adding pump requirement | Increase FA% by 2–4% | Pump mixes need more mortar (cement + FA + water) to lubricate the pipeline | 2–4% |
| SCM Addition | Adding 20% Fly Ash | Can allow slight increase | FA ball-bearing effect improves workability; may allow slightly higher FA% | 0–2% |
| High Workability | Slump increase 75→150mm | Slight FA increase | Higher workability mixes benefit from more FA for cohesion; prevents segregation | 1–2% |
Q: What FA:CA ratio should I use for M30 concrete with 20mm aggregate and Zone II sand?
Per IS 10262:2019 Annex A Table A-1 for 20mm aggregate, Zone II sand, and w/c ≈ 0.45 (typical for M30 OPC 53): FA% = 34–38% of total aggregate volume. Use 36% as the starting point for trial mix. If actual FM of the Zone II sand is > 3.0, increase to 38%. If FM is < 2.7, reduce to 34%. Confirm with trial mix slump and visual assessment — the mix should be cohesive without being sticky.
Q: Why does IS 10262 use % of total aggregate volume rather than mass ratio?
Because FA and CA typically have similar but not identical specific gravities. Using volume percentage ensures the physical packing and void-filling geometry is correctly captured. If you use mass percentage and the SG of FA (2.65) differs from CA (2.68), the actual volume split will be slightly different from the mass split — enough to matter in precision mix design. The volume-basis approach also directly feeds into the absolute volume balance calculation without any SG conversion step.
Q: How do I handle aggregate proportioning when using both 20mm and 10mm coarse aggregate?
Determine total CA volume from the absolute volume balance (V_CA = V_agg × (1−FA%)). Then split this CA volume (or mass) in the ratio 60:40 (20mm:10mm) per IS 10262:2019 recommendation — or 65:35 for mixes with Zone I or Zone II sand at higher w/c ratios. For HPC M50+ mixes, a 70:30 blend is sometimes used to maximise packing density. Calculate SSD masses and absorption separately for each CA fraction and apply individual moisture corrections at batching.
Q: Can I use aggregate proportioning to fix a mix that is too stiff without adding water?
Yes — if the mix is too stiff (low slump) without adding water, first check if the FA:CA ratio is correct. If FA% is too low (e.g. 28% for Zone II 20mm at w/c 0.50 where 38% is recommended), increasing FA% to the correct value improves workability without any water addition — because FA particles fit better between CA particles, reducing friction and improving flow. This is the correct approach. Adding water to fix a stiff mix increases w/c and permanently damages the concrete's strength and durability.
Q: What is the aggregate proportioning method in MixDesignCalc?
MixDesignCalc implements IS 10262:2019 Annex A Table A-1 for FA% selection based on aggregate size, FA zone, and w/c ratio. It then applies the FM correction (±1.5% per 0.1 FM deviation) if actual FM differs from the zone design FM. The absolute volume balance computes FA and CA masses using V_FA = V_agg × FA% and V_CA = V_agg × (1−FA%), with SSD masses = Volume × SG × 1000. Two-CA blending is a separate optional tool that distributes the total CA mass between 20mm and 10mm fractions.