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USG Hydrocal White Calculator: Mix Ratio and Batch Size

Hard gypsum cement for durable casts and figurines.

Your measurements

Material

1.762 g/cm³

Mix 100:42.5 by weight. USG lists a hand-mix Vicat set target of 20–30 minutes.

Mold

How to measure the mold

Enter the inside size of the mold box.

Sets the volume formula. For an irregular cavity, weigh water instead.

How deep the material will be, not the height of the box walls.

The space the original object takes up inside the box. Find it by dunking it in a measuring jug of water. Use 0 for an empty mold.

Extra for waste

Material that stays on the cup walls and stirrer. 8% is a common starting point; use more for thick mixes.

Common buffers

Total to mix1427.2 g

Results

Total USG Hydrocal White Gypsum Cement to mix

1427.2g

= 50.34 oz (3.15 lb)

Mixed weight with 8% extra for waste, at 1.762 g/cm³. Weigh each part on a scale.

Hydrocal
1001.6g
= 35.33 oz
100 parts by weight
Water
425.7g
= 15.01 oz
42.5 parts by weight
Cavity volume
750mL
= 25.36 fl oz
Before the buffer
Batch volume
810mL
= 27.4 fl oz
With the buffer
Measuring USG Hydrocal White Gypsum CementHydrocal: 1001.6 g (35.33 oz). Water: 425.7 g (15.01 oz), about 426 mL in a 500 mL cup. Mix ratio 100:42.5 by weight.100:42.5 by weight500 mL cupweigh itHydrocal70% of the mix by weight1001.6 g (35.33 oz)100200300400500mLWater30% of the mix by weight425.7 g (15.01 oz)+
Levels are shown in a 500 mL cup. Weigh each part on a scale that reads in grams; the levels are a guide. Parts marked “weigh it” have no reliable volume, so no level is drawn.

100:42.5 by weight. Weigh water and Hydrocal, sift the powder onto the water, let it soak, then mix.

Mold box

Inside size of the box with your numbers.

Mold box sectionSection along a 15 cm · 5.91" long, 10 cm · 3.94" wide mold box filled 6 cm · 2.36" deep around a 150 cm³ master. Net pour 750 mL of USG Hydrocal White Gypsum Cement.master 150 cm³pour 750 mLInside length 15 cm · 5.91"Depth 6 cm · 2.36"
Section along the length. Inside width 10 cm · 3.94". The master is drawn as a block; its real shape does not change the volume.

How this was calculated

Volume, then weight, then the split, with your numbers.

  1. Step 1: Volume to fill

    V = L × W × H − V_master

    = 15 × 10 × 6 − 150 cm³
    = 750 cm³ (mL)

    The inside of the mold minus the 150 cm³ the master takes up.

  2. Step 2: Weight with a 8% buffer

    W = V × density × (1 + buffer)

    = 750 cm³ × 1.762 g/cm³ × 1.08
    = 1427.2 g

    1.762 g/cm³ is the mixed density from the maker's data. The buffer covers what stays in the cup and on the stirrer.

  3. Step 3: Split by the weight ratio

    A = W × R_A ÷ (R_A + R_B)

    = 1427.2 g × 100 ÷ 142.5
    = 1001.6 g Hydrocal + 425.7 g Water

    100:42.5 by weight.

Sources

In short

USG Hydrocal White: 100:42.5 by weight. A 15 × 10 × 6 cm rectangular mold box with a 150 cm³ master needs about 1427 g: 1001.6 g Hydrocal + 425.7 g Water.

Mixed 100 parts Hydrocal to 40–45 parts water by weight. The calculator loads its density (1.762 g/cm³) and ratio (100:42.5 by weight); choose your mold shape and size.

USG Hydrocal White data

PropertyValue
Mix ratio100:42.5 by weight
PartsHydrocal + Water
Mixed density1.762 g/cm³ (USG lists a wet density of 110 lb/ft³ (1762 kg/m³).)
Pot lifeSee notes
DemoldSee notes
Full cureSee notes
Timing noteUSG lists a hand-mix Vicat set target of 20–30 minutes.

How much USG Hydrocal White for common molds

Each row assumes a 150 cm³ master inside the mold and a 8% buffer. Pick the shape in the calculator and enter your own sizes for your mold.

MoldSizeTo fillTotalSplit
Rectangular Mold Box15 × 10 × 6 cm750 cm³1427 g1001.6 g Hydrocal + 425.7 g Water
Cylindrical Cup MoldØ9 × 12 cm613.4 cm³1167 g819.2 g Hydrocal + 348.1 g Water
Shallow Tray / Coaster Mold20 × 15 × 2 cm450 cm³856 g600.9 g Hydrocal + 255.4 g Water
Hemisphere / Dome MoldØ10 cm111.8 cm³213 g149.3 g Hydrocal + 63.5 g Water
Custom Geometric Block12 × 12 × 8 cm1002 cm³1907 g1338.1 g Hydrocal + 568.7 g Water

Casting materials compared

Mixed density sets the weight you need; the ratio sets how to split it. Generic epoxy entries are typical values, so use your kit's label if it differs.

MaterialDensityMix ratioPot lifeDemoldBasis
Mold Star 30 Silicone1.12 g/cm³100:96 by weight, 1:1 by volume45 min6 hMaker's data sheet
Mold Max 30 Silicone1.18 g/cm³100:10 by weight45 min24 hMaker's data sheet
Jesmonite AC1001.845 g/cm³2.5:1 by weightSee notesSee notesMaker's data sheet
Jesmonite AC7301.95 g/cm³5:1 by weightSee notesSee notesMaker's data sheet
Deep-Pour Epoxy (generic 2:1 by volume)1.11 g/cm³2:1 by volumeSee notesSee notesTypical (estimate)
Table-Top Epoxy (generic 1:1 by volume)1.08 g/cm³1:1 by volumeSee notesSee notesTypical (estimate)
Smooth-Cast 300 Resin1.05 g/cm³100:90 by weight, 1:1 by volume3 min10 minMaker's data sheet
Dragon Skin FX-Pro1.062 g/cm³1:1 by weight or volume12 min40 minMaker's data sheet
USG No. 1 Pottery Plaster1.58 g/cm³ (est.)100:70 by weightSee notesSee notesMaker's data sheet
USG Hydrocal White1.762 g/cm³100:42.5 by weightSee notesSee notesMaker's data sheet
Clear Flex 50 Urethane1.04 g/cm³1:2 by weight25 min16 hMaker's data sheet

Worked example: USG Hydrocal White in a rectangular mold box

The three steps the calculator runs: volume, weight, split.

  1. Step 1: Volume to fill

    V = L × W × H − V_master

    = 15 × 10 × 6 − 150 cm³
    = 750 cm³ (mL)

    The inside of the mold minus the 150 cm³ the master takes up.

  2. Step 2: Weight with a 8% buffer

    W = V × density × (1 + buffer)

    = 750 cm³ × 1.762 g/cm³ × 1.08
    = 1427.2 g

    1.762 g/cm³ is the mixed density from the maker's data. The buffer covers what stays in the cup and on the stirrer.

  3. Step 3: Split by the weight ratio

    A = W × R_A ÷ (R_A + R_B)

    = 1427.2 g × 100 ÷ 142.5
    = 1001.6 g Hydrocal + 425.7 g Water

    100:42.5 by weight.

Working with USG Hydrocal White

Starting points only. Your machine, material and room conditions may differ.

Most common mistake

Using the pottery-plaster ratio (70 parts water). Hydrocal needs far less water.

Tips

  • USG gives a normal consistency of 40–45 parts water per 100 parts Hydrocal by weight; this calculator uses the midpoint, 42.5.
  • Hydrocal White is much harder than pottery plaster (USG lists 6000 psi dry compressive strength).
  • Use water between 70°F (21°C) and 100°F (38°C), as USG recommends.

Typical settings

Mixing technique
Weigh water and Hydrocal, sift the powder onto the water, let it soak, then mix.
Degassing required
No
Demold note
Hand-mix Vicat set target is 20–30 minutes.

USG Hydrocal White: common questions

Ratio, batch size and timing.

What is the mix ratio of USG Hydrocal White?

100 parts Hydrocal to 42.5 parts Water by weight. Source: USG Hydrocal White Gypsum Cement data sheet (IG1381).

How much USG Hydrocal White do I need?

Multiply the volume to fill by 1.762 g/cm³ and add a little extra. For example, a rectangular mold box of 15 × 10 × 6 cm with a 150 cm³ master inside needs about 1427 g (1001.6 g Hydrocal + 425.7 g Water) with an 8% buffer.

How long is the working time and demold time of USG Hydrocal White?

USG lists a hand-mix Vicat set target of 20–30 minutes.