Drywall Calculations

How to Calculate Core Drywall Line Metrics: Slurry, Speed, Scrap, and Moisture

A worked walkthrough of the core gypsum board formulas, from line speed and slurry mass to paper consumption, scrap, and moisture balance, with real units and numbers.

Start with board line speed, the number that scales every other input. A modern gypsum line runs 300 to 500 linear feet per minute (ft/min). Convert to boards per hour by dividing 60 times speed by board length: at 400 ft/min on 12 ft boards you get 60 x 400 / 12 = 2,000 boards per hour. On 8 ft boards the same speed yields 3,000 boards per hour. Multiply by 4 ft width to get face area: 2,000 boards x 96 ft2 = 192,000 ft2 per hour, or about 1.5 million square feet on an 8 hour shift with 80 percent uptime. The Board Line Speed calculator handles the length and width conversions for you.

Gypsum slurry usage follows directly from board volume and target core density. A standard 1/2 inch board has a core density near 1,700 lb per thousand square feet (MSF). At 192,000 ft2 per hour that is 192 MSF x 1,700 = 326,400 lb of dry core solids per hour, roughly 163 tons. Stucco (calcined gypsum) is typically 90 to 94 percent of that dry mass, with the balance being starch, accelerator, dispersant, and paper fiber. Water is added at a water-to-stucco ratio of 0.75 to 1.0 by weight, so 326,000 lb of stucco pulls 245,000 to 326,000 lb of gauge water. The Gypsum Slurry Usage calculator ties density, thickness, and speed into a mass flow.

Paper facing consumption is a length-and-width problem plus a wrap allowance. Face and back paper each cover the full board plus the folded edges. For a 4 ft wide board the flat face paper is about 48.5 inches wide to allow the 1/4 inch tapered edge wrap on each side, and back paper runs near 49.5 inches. At 400 ft/min, each roll feeds 400 ft/min x 60 = 24,000 linear feet per hour per web. Multiply by basis weight: face paper near 45 lb per MSF and back near 40 lb per MSF gives roughly 192 MSF x 85 lb combined = 16,300 lb of paper per hour. Paper Facing Consumption converts web width, basis weight, and speed into pounds and rolls.

Scrap board rate is measured as rejected board area divided by total board area produced. If a shift produces 1.5 million ft2 and the knife-to-stacker rejects 42,000 ft2 for wet ends, bond failures, and dimension faults, scrap is 42,000 / 1,500,000 = 2.8 percent. Track it in MSF so it maps to material loss: 2.8 percent of 1,500 MSF is 42 MSF, and at 1,700 lb per MSF that is 71,400 lb of gypsum core plus its paper gone to the crusher. The Scrap Board Rate calculator separates head-and-tail trim from in-process rejects so you do not double count.

Edge trim loss is a width calculation, not a rejection. Boards are formed slightly wide and the wet edges are slit before the dryer. If forming width is 49 inches and finished width is 48 inches, trim loss is (49 minus 48) / 49 = 2.04 percent of throughput, continuous and unavoidable within a band. Combine trim with scrap for total yield loss: 2.0 percent trim plus 2.8 percent scrap gives 4.8 percent, so first-pass yield is 95.2 percent. Use the Edge Trim Loss calculator to convert the width delta into MSF and gypsum tons per shift.

The moisture balance across the dryer sets both quality and energy. Freshly formed board leaves the setting belt at 40 to 45 percent free water by wet weight; finished board must hit 0.5 to 1.0 percent. Water to evaporate per MSF is board weight times the moisture delta. For a 1/2 inch board near 2,100 lb per MSF wet, evaporating from 42 percent to 1 percent removes about 2,100 x (0.42 minus 0.01) = 861 lb of water per MSF. Across 192 MSF per hour that is 165,000 lb, roughly 20,000 gallons of water evaporated hourly. The Moisture Target Window calculator flags board that will over-dry or exit wet.

Dryer energy cost comes from that evaporation load plus efficiency losses. The latent heat of water is about 970 BTU per pound, so 165,000 lb per hour needs 160 million BTU per hour of useful heat. Real dryers run 60 to 70 percent thermal efficiency, so gross fuel input is closer to 160 / 0.65 = 246 million BTU per hour. At natural gas of 6 dollars per MMBTU that is 246 x 6 = 1,476 dollars per hour of drying fuel. Feed this into the Dryer Energy Cost calculator alongside your line speed to get cost per MSF.

Close the loop with stacker and packaging math so the front-end numbers reconcile with what ships. Boards are cut to length, and a stacker collects them into units. If a unit holds 40 boards of 12 ft by 1/2 inch and the line makes 2,000 boards per hour, that is 50 units per hour, one every 72 seconds, which must clear before the next arrives. Packaging count then bundles two boards per pair with tape and end caps. Use the Stacker Capacity and Packaging Count calculators to confirm the downstream cadence matches your 2,000 board per hour front end, or the buffer overflows.

Published 2026-07-01.