Masterbatch Let-Down Ratio: How to Calculate Your Dosage

The let-down ratio is how much masterbatch you add to natural polymer, and getting it wrong is the most common reason a shade comes out short, a UV package underperforms, or a costing goes sideways.

The arithmetic is simple. The mistakes are not.

The two numbers people confuse

Dosage is the percentage of masterbatch in the total mix. 4% dosage means 4 kg of masterbatch in every 100 kg of finished compound.

Let-down ratio (LDR) is the same information written as a ratio – 1 part masterbatch to N parts natural polymer.

LDR = (100 ÷ dosage %) − 1, expressed as 1 : N

At 4% dosage, that is 100 ÷ 4 = 25, minus 1 = 1 : 24. One kilo of masterbatch to twenty-four kilos of natural resin.

You will see 1:25 quoted for 4% as often as 1:24, because some suppliers write the ratio as masterbatch-to-total rather than masterbatch-to-natural. Both are used in the trade. When it matters, confirm which convention your supplier means – at high dosages the difference is real money.

The mistake that costs the most: dosing the active percentage

This is the big one, and it turns up constantly.

A specification says “the part must contain 2% titanium dioxide“. The operator sets the dosing unit to 2%. The part comes out under-covered, and everyone blames the masterbatch.

The masterbatch is not 100% TiO₂. If it is a 60% white, then dosing 2% of it puts 1.2% TiO₂ into the part, not 2%.

Required masterbatch dosage % = (required active % in part ÷ active % in masterbatch) × 100

For 2% TiO₂ from a 60% white masterbatch: (2 ÷ 60) × 100 = 3.33% dosage.

The same trap applies to carbon black against IS 4984, to UV stabiliser dosages quoted in ppm, and to any additive specified as a concentration in the finished article rather than as a hopper setting.

Always ask your supplier which number they are quoting: the addition rate, or the active content. A good technical data sheet states both.

Typical dosage ranges

These are working starting points, not specifications. Actual dosage depends on pigment strength, part thickness, opacity target and the polymer.

Application	Typical dosage
Colour masterbatch, standard opaque part	1 – 4%
Colour masterbatch, deep or difficult shades	4 – 8%
Black masterbatch, general purpose appearance	1 – 3%
Black masterbatch, UV-protected outdoor (pipe, geomembrane)	5 – 8%
White masterbatch, thin film	2 – 6%
White masterbatch, thick section or high opacity	4 – 10%
UV stabiliser masterbatch, agricultural film	2 – 7%, set by required kLy lifetime
Filler masterbatch	10 – 30%

Deep shades cost more not because the pigment is exotic but because you need more of it. A 6% dosage on a rich blue is not a supplier failing – it is what the shade requires.

Thickness changes everything

Colour is a light-path phenomenon. The same dosage in a 200-micron sheet looks entirely different from a 25-micron film, because light travels eight times further through the pigment in the thicker part.

Which means: a dosage recommendation is meaningless without a thickness. If you move a shade from a 2 mm injection-moulded part to a 40-micron film, you will need substantially more masterbatch to hold the same colour, and probably a reformulation rather than a dosage change.

When you brief a supplier for a colour match, give them the wall thickness and the process. Without those, they are guessing.

Costing it properly

The number that matters is not the price per kilo of masterbatch. It is the cost per kilo of finished compound.

Added cost per kg of compound = masterbatch price per kg × dosage %

A ₹180/kg masterbatch at 2% dosage adds ₹3.60 per kg of compound. A ₹120/kg masterbatch at 4% dosage adds ₹4.80 per kg of compound.

The cheaper masterbatch is 25% more expensive in use. This is the single most common way a purchase decision goes wrong in this industry – and it is why a supplier quoting a higher price per kilo may still be the cheaper supplier.

Ask every quote for a recommended dosage and compare on cost-in-use, not price-per-kilo.

Dosing accuracy in practice

Two more things that decide whether the calculated number is the number you actually get:

Volumetric versus gravimetric dosing. A volumetric feeder meters by screw revolutions and assumes a constant bulk density. Masterbatch and natural resin rarely have the same bulk density, and pellet shape varies between lots. Gravimetric dosing weighs, so it holds the set point when density drifts. If your shade wanders and your masterbatch tests fine, look at the feeder before you blame the supplier.

Separation in the hopper. Where the masterbatch pellet size and density differ significantly from the natural resin, the blend can separate as it settles – giving you rich colour at the start of a hopper and weak colour at the end. Pellet size matching is a real quality parameter, and it is worth asking about.

FAQs

How do I calculate the let-down ratio? Divide 100 by your dosage percentage and subtract 1. At 3% dosage: 100 ÷ 3 = 33.3, minus 1 = 1 : 32.3. Confirm whether your supplier quotes the ratio against natural resin or against total compound.

What is a typical masterbatch dosage? Between 1% and 4% for most colour applications. Deep shades, high-opacity whites and UV-protected outdoor products run higher – 5% to 8% is normal for UV-grade black.

Why is my colour weak at the correct dosage? Most often because the dosage was set to the required pigment percentage rather than the required masterbatch percentage. Divide the target pigment percentage by the pigment loading in the masterbatch. Other causes are volumetric feeder drift and a thinner wall section than the shade was matched at.

Does a higher dosage always mean a better colour? No. Past the point of full opacity you are adding cost without adding colour, and at high loadings you can affect mechanical properties, processing and surface finish.

How do I compare two masterbatch quotes fairly? On cost in use, not price per kilo. Multiply each price per kilo by its recommended dosage to get the added cost per kilo of finished compound.

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