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2026-11-06T09:15:58

Concrete Strength Class K-225, K-250, K-300 and fc' under SNI 2847

混凝土抗压等级K-225、K-250、K-300与SNI 2847的fc'

The concrete strength class K-225, along with K-250 and K-300, still appears in bills of quantities and site talk, yet SNI 2847:2019 specifies fc' in MPa from cylinder specimens. The K figure is the characteristic cube strength (15 cm cubes, kg/cm²) from PBI 1971. With the 0.83 shape factor, K-225 ≈ 18.3 MPa, K-250 ≈ 20.3 MPa and K-300 ≈ 24.4 MPa; the SNI 7394:2008 pairing table lists 19.3, 21.7 and 26.4 MPa. This article explains the codes, when each fits houses versus structural frames, and how to read a simple material schedule.

Key Facts

What concrete strength class K-225, K-250 and K-300 mean

The letter K comes from Indonesia's 1971 reinforced-concrete code (PBI 1971). The number is the characteristic compressive strength of 15 cm cubes at 28 days, in kg/cm², with at most a 5% chance of test results falling below it. K-225 therefore means 225 kg/cm² on that cube.

SNI 2847:2019 (structural concrete for buildings) does not use K. It uses fc', the specified compressive strength in MPa, proven on 150×300 mm cylinders (or three 100×200 mm cylinders) at 28 days. One "test result" is the average of two cylinders from the same sample.

How to convert K to fc' (MPa)

Conversion needs two steps: change units (1 kg/cm² = 0.0980665 MPa) then correct for specimen shape. PBI 1971 Table 4.1.3 gives a 0.83 factor for a 15×30 cm cylinder versus a 15 cm cube. Approximate formula: fc' ≈ 0.83 × 0.0980665 × K ≈ 0.0814 × K. For the three grades most used in houses and shophouses:

  • K-225 → ≈ 18.3 MPa (SNI 7394 pairing: 19.3 MPa)
  • K-250 → ≈ 20.3 MPa (SNI 7394 pairing: 21.7 MPa)
  • K-300 → ≈ 24.4 MPa (SNI 7394 pairing: 26.4 MPa)

Do not simply divide K by 10: that skips the shape factor and slightly overstates the unit conversion. For ready-mix orders and acceptance records, write fc' and prove it with cylinders.

When to use each grade in houses vs structure

SNI 2847:2019 Table 19.2.1.1 sets a general minimum of 17 MPa and raises it to 21 MPa for special moment frames and special structural walls. In practice:

  • K-225 (≈18–19 MPa): blinding, pavements or non-critical house elements when the drawing asks for this grade. Above 17 MPa, but below 21 MPa for special frames.
  • K-250 (≈20–22 MPa): a grey zone. The older AHSP pairing gives 21.7 MPa; the 0.83 factor gives only 20.3 MPa. If the drawing asks for fc' 21 MPa, order "fc' 21 MPa" directly.
  • K-300 (≈24–26 MPa): columns, beams and floor slabs in multi-storey houses and shophouses designed above 21 MPa.

The current AHSP under Permen PUPR 8/2023 lists concrete items only in fc' (for example 20, 25, 30 MPa) without a K pairing. Old bills that still use K need conversion before ordering.

Reading a simple mix / material schedule

SNI 7394:2008 coefficients (slump 12±2 cm, max aggregate 30 mm) give a rough per-m³ guide, not the site mix design:

  • K-225: 371 kg cement (~7.4 bags), 698 kg sand, 1,047 kg gravel, 215 L water
  • K-250: 384 kg cement (~7.7 bags), 692 kg sand, 1,039 kg gravel, 215 L water
  • K-300: 413 kg cement (~8.3 bags), 681 kg sand, 1,021 kg gravel, 215 L water

Official mix design follows SNI 7656:2012 or SNI 03-2834-2000 and is proven by cylinder tests. Without field standard-deviation data, SNI 03-2834-2000 targets at least fc' + 12 MPa average. After casting, acceptance depends on test results, not the K conversion.

homeX, an Indonesian one-stop building materials and furniture platform with showroom, warehousing and delivery, can help compile cement, sand, gravel and rebar lists to match the grade written on the drawings.

concrete strength class K-225: related articles and products

FAQ

How many MPa is K-300? About 24.4 MPa with the PBI 1971 0.83 factor; SNI 7394:2008 pairs it with fc' 26.4 MPa.

Is K-225 enough for house columns? K-225 ≈ 18–19 MPa, above the 17 MPa general minimum but below 21 MPa for special frames. Follow the fc' on the structural drawing.

Why not just divide K by 10? Dividing by 10 only roughly changes units and does not correct cube strength to cylinder strength.

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Sources: Ruang Teknik, Concrete grade K and fc': meaning, conversion and examples (updated 19 Oct 2026); CivPlanner, Concrete grade conversion K to f'c (data checked 3 Oct 2026)