How do you calculate shot weight?

Multiply resin weight per cavity by the number of cavities, then apply a shot allowance for the runner and sprue. With 28 g per cavity across 8 cavities and a 1.05 allowance, the cavity resin weight is 224 g and the total shot weight is 235.2 g per cycle.

What is the difference between shot weight and part weight?

Part weight is one finished molded piece. Shot weight is everything injected in a cycle: all cavities plus the cold runner and sprue. On an 8-cavity tool, shot weight is roughly eight part weights plus the runner system, which is what the 1.05 allowance captures here.

What is a good shot-to-barrel ratio?

Aim to use 20 to 80 percent of the barrel's rated shot capacity, with 40 to 60 percent ideal. Below 20 percent the resin sits too long and degrades; above 80 percent you lose cushion control and consistent fill. Size the press so 235.2 g lands comfortably in that window.

Why does shot weight matter for resin cost?

Shot weight times cycles per hour is your hourly resin draw. The runner and sprue in the shot allowance are resin you pay for every cycle and either regrind or scrap, so trimming runner mass directly lowers cost per part on high-volume preform and bottle production.

How do I reduce the shot allowance multiplier?

Shrink the cold runner cross-section, shorten runner length, or move to a hot runner that eliminates the runner from the shot entirely. Going from a 1.05 to a 1.02 allowance on a 224 g cavity charge saves about 7 g of regrind every cycle.

What happens if shot weight is set too low?

Too little shot weight starves the cavities and produces short shots, sinks, and weak welds, often worst in the cavity farthest from the gate. Increasing shot size or transfer position restores fill, but watch that you do not flip into over-packing and flash at the parting line.

Blow Molding & Hollow Plastic Products calculator

Shot Weight Calculator

Shot weight is the total resin an injection or injection-blow machine plasticizes and injects in one cycle to fill every cavity plus the runner and sprue overhead. Setters use it to confirm the barrel can deliver the required volume, to set cushion and shot size on the screw, and to check that a part is being filled to spec rather than packed short. On a multi-cavity preform tool it is the single most-watched number for both quality and resin cost. Knowing it precisely keeps you off the edge of short shots and away from over-packing flash.

What this calculator does

Estimate total shot or drop weight for injection blow, stretch blow, or extrusion blow molded containers from preform or parison weight, cavities, and allowance.
a blow molding processor needs to estimate total resin shot weight for a bottle, preform, or hollow part mold
It multiplies resin weight per cavity by the number of cavities filled per shot, then applies a shot allowance for runners and sprue and a unit-conversion factor to return total shot weight.

Formula used

Cavity resin weight per cycle = resin weight per cavity × active cavities per shot
Total shot weight = cavity resin weight per cycle × shot allowance multiplier × unit conversion multiplier

Inputs explained

Resin weight per cavity:
Active cavities per shot:
Shot allowance multiplier:
Unit conversion multiplier:

How to use the result

Use it when setting shot size on a new mold, troubleshooting short shots or flash, or estimating resin draw per cycle for cost and capacity planning.
It treats every cavity as filling equally; on imbalanced runner systems or worn tooling, real per-cavity weights vary and a single average can mask a starved cavity.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 319.371 (BLS, May 2026), up 19.5% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate shot weight? Multiply resin weight per cavity by the number of cavities, then apply a shot allowance for the runner and sprue. With 28 g per cavity across 8 cavities and a 1.05 allowance, the cavity resin weight is 224 g and the total shot weight is 235.2 g per cycle.
What is the difference between shot weight and part weight? Part weight is one finished molded piece. Shot weight is everything injected in a cycle: all cavities plus the cold runner and sprue. On an 8-cavity tool, shot weight is roughly eight part weights plus the runner system, which is what the 1.05 allowance captures here.
What is a good shot-to-barrel ratio? Aim to use 20 to 80 percent of the barrel's rated shot capacity, with 40 to 60 percent ideal. Below 20 percent the resin sits too long and degrades; above 80 percent you lose cushion control and consistent fill. Size the press so 235.2 g lands comfortably in that window.
Why does shot weight matter for resin cost? Shot weight times cycles per hour is your hourly resin draw. The runner and sprue in the shot allowance are resin you pay for every cycle and either regrind or scrap, so trimming runner mass directly lowers cost per part on high-volume preform and bottle production.
How do I reduce the shot allowance multiplier? Shrink the cold runner cross-section, shorten runner length, or move to a hot runner that eliminates the runner from the shot entirely. Going from a 1.05 to a 1.02 allowance on a 224 g cavity charge saves about 7 g of regrind every cycle.

Last reviewed 2026-05-12.