What this membrane estimate measures
Finished-system coverage represents the area completed with one package under the selected application conditions. It differs from the area a single coat can cover.
Waterproofing and crack prevention meeting ANSI A118.12 have different captured coverage rows. Choosing a row is a quantity assumption, not evidence that the project meets a standard or that the product is suitable.
Sheet mode counts the first full strip width, then each additional strip contributes roll width minus side overlap. Full cut lengths must fit inside individual rolls. Total nominal roll area alone can overstate useful material when remnants cannot make another complete strip.
Choose the strip orientation yourself. Allowance in sheet mode adds to each strip’s cut length only; it does not increase the measured span or prescribe a different lap. This keeps the side-lap geometry explicit.
Liquid mode retains the captured RedGard finished-system rows. Sheet mode models one flat rectangle covered by one-piece KERDI strips with at least 2 in side laps and confirmed roll dimensions. No end splices, banded butt joints, corners, penetrations, accessories or waterproofing design are included.
How to use this calculator
- Membrane format. Choose the existing liquid coverage model or the sheet-strip model. Switching formats clears the estimate and disables irrelevant controls.
- Project area. Measure the area to receive the selected membrane application in square feet. Include only surfaces using the same product, purpose and package.
- Application and package coverage. Select the finished waterproof-system row or the crack-prevention row specifically labeled as meeting ANSI A118.12, with the correct 1 or 3.5 US gallon container. The general 100/350 ft² crack-prevention rows are not used here.
- Rectangle span across strips / Full strip run. For sheets, enter one flat rectangle’s span across strips and run along the strip. The model does not rotate the layout, combine surfaces or join short lengths end-to-end.
- Confirmed roll width / Confirmed usable roll length / Confirmed side overlap. Enter confirmed roll width, usable roll length and specified side overlap. KERDI direct laps require at least 2 in here. Overlap must be smaller than roll width, and a full strip including allowance must fit within one roll.
- Waste / allowance. The optional allowance applies once to area in liquid mode and once to each full strip’s cut length in sheet mode. It is not an additional coat or side-lap multiplier. Zero adds none.
Select Calculate to update the result. Reset restores the example inputs and recalculates the example estimate.
Formulas, variables, and units
adjustedAreaSqFt = areaSqFt × (1 + wasteRate / 100)
- areaSqFt
- Measured area for the selected completed application.
- wasteRate
- Explicit additional percentage; zero adds none.
- adjustedAreaSqFt
- Area after applying the allowance once.
requiredContainers = adjustedAreaSqFt / coverageSqFtPerContainer; containers = ceil(requiredContainers)
- coverageSqFtPerContainer
- Selected finished-system coverage for the exact package, in square feet.
- requiredContainers
- Fractional package requirement.
- containers
- Complete containers of that size, rounded upward without early decimal rounding.
widthFt = rollWidthIn / 12; lapFt = overlapIn / 12; strips = max(1, ceil((spanFt − lapFt) / (widthFt − lapFt)))
- spanFt, runFt
- The chosen rectangle dimensions across and along strips, in feet.
- rollWidthIn, overlapIn
- Confirmed roll width and direct side lap, in inches. Lap is at least 2 in and strictly less than width.
- strips
- Complete one-piece strips needed to cover the measured span.
cutFt = runFt × (1 + wasteRate / 100); stripsPerRoll = floor(rollLengthFt / cutFt); rolls = ceil(strips / stripsPerRoll)
- cutFt
- Full strip cut length including the explicit allowance.
- rollLengthFt
- Confirmed usable length of one roll, in feet.
- floor, stripsPerRoll
- Round downward to complete full-length cuts obtainable from one roll; no end splices assumed.
- rolls
- Whole rolls after considering these complete cuts, not just aggregate area.
coveredSpanFt = strips × widthFt − (strips − 1) × lapFt; cutLengthFt = strips × cutFt; purchasedLengthFt = rolls × rollLengthFt
- coveredSpanFt
- Nominal width covered by the full strips after side laps.
- cutLengthFt
- Total linear length of the modeled full strip cuts.
- purchasedLengthFt
- Total linear length in complete purchased rolls; leftovers are not guaranteed useful.
How to read your result
Adjusted area is the measured area plus the explicit allowance.
Complete containers refer to the selected package only. The 3.5 gallon waterproof row is 192 ft² as captured, not 3.5 × 55 rounded or rescaled.
Nominal purchased coverage multiplies the whole-container count by that fixed row. It does not prove wet/dry film thickness, waterproof continuity or installation compliance.
Sheet results show full strips, cut length with allowance, complete cuts per roll and whole rolls. Short remnants are not spliced; the result can exceed a pure area or total-length division.
Nominal covered span and purchased roll length describe this chosen layout only. They do not include bands, corners, drains, penetrations or installation-system compliance.
Displayed values use up to two decimal places. Complete purchase units are rounded upward from the unrounded requirement.
A worked purchase example
- Membrane format
- Liquid membrane — captured RedGard rows
- Project area
- 140 sq ft
- Application and package coverage
- RedGard waterproof membrane — 1 US gal container — 55 sq ft finished system
- Waste / allowance
- 10 %
- For 140 ft² and an elected 10% allowance, adjusted area is 140 × 1.10 = 154 ft².
- Select the RedGard 1 US gallon waterproof finished-system row: 55 ft² per container.
- Fractional demand is 154 / 55 = 2.8 containers; round upward to 3 one-gallon containers.
- Nominal purchased coverage is 3 × 55 = 165 ft². No additional coat multiplier is applied.
Independent arithmetic using the captured February 2025 finished-system row. Film thickness, preparation and suitability require separate confirmation.
Full strips with side laps and cut-length allowance
- Membrane format
- Sheet membrane — one-piece KERDI strips with side laps
- Rectangle span across strips
- 8 ft
- Full strip run
- 10 ft
- Confirmed roll width
- 39 in
- Confirmed usable roll length
- 33 ft
- Confirmed side overlap
- 2 in
- Waste / allowance
- 10 %
- For a hypothetical confirmed 39 in roll, width is 39/12 = 3.25 ft. A 2 in side lap is 1/6 ft.
- For an 8 ft span, strips = ceil((8 − 1/6)/(3.25 − 1/6)) = ceil(94/37) = 3. Their nominal covered span is 3 × 3.25 − 2 × (1/6) = 9.4166666667 ft.
- A 10 ft run with elected 10% cut-length allowance requires 11 ft per full strip. A 33 ft usable roll provides floor(33/11) = 3 cuts, so 3 strips require 1 roll.
- The model cuts 3 × 11 = 33 linear ft and buys 33 linear ft. No additional side-lap or coat multiplier is added.
Independently derived example using explicitly entered, hypothetical confirmed dimensions. Catalogue metric/customary rounding must not be treated as exact interchangeable lengths.
Why total length is not enough when splices are excluded
- Membrane format
- Sheet membrane — one-piece KERDI strips with side laps
- Rectangle span across strips
- 10 ft
- Full strip run
- 6 ft
- Confirmed roll width
- 36 in
- Confirmed usable roll length
- 10 ft
- Confirmed side overlap
- 2 in
- Waste / allowance
- 0 %
- For a hypothetical 36 in width and 2 in side lap, strips = ceil((10 − 1/6)/(3 − 1/6)) = ceil(59/17) = 4.
- Each strip is 6 ft and each hypothetical usable roll is 10 ft. A roll provides only floor(10/6) = 1 complete strip.
- Four strips therefore require 4 rolls:24 ft cut and 40 ft purchased. Dividing total 24 ft by 10 ft would suggest 3 rolls, but their 4 ft remnants cannot make a 6 ft unspliced strip.
Independent cut-capacity boundary example, not a stock KERDI package or an optimized installation plan.
Frequently asked questions
Should I multiply by two for two coats?
No. These are captured finished-system coverage rows. Adding a coat multiplier would count the system coverage twice; application requirements still need separate confirmation.
Why is crack-prevention coverage lower than some other charts?
This selector uses the specifically labeled ANSI A118.12 rows of 25 and 88 ft², rather than the separate general crack-prevention rows of 100 and 350 ft². The conditions are not interchangeable.
Does the estimate verify waterproofing?
No. It counts packages for a declared application row. Preparation, film thickness, transitions, curing and suitability must be addressed independently.
Can sheet mode join short remnants or use banded butt seams?
No. It uses one-piece full-run strips with direct side laps. End splices, butt joints with bands, corners and accessories require separate layout and material estimates.
Why can I need more rolls than total area suggests?
Each strip must fit unspliced within a roll. Short remnants may add up to enough area or length but still fail to provide another complete strip.
What does the sheet allowance change?
It increases each strip’s cut length only. It does not alter the measured span or side overlap. A cut longer than one usable roll is rejected.
Does this choose the best orientation or verify waterproofing?
No. You choose the orientation and confirm roll size, lap and system. The quantity model does not optimize rotation, design corners or certify installation.
Scope and limitations
Project types
- Interior waterproof membrane area
- Interior crack-prevention area with specified ANSI A118.12 application
Check the calculator assumptions and product conditions for your project.
Calculation limits
- Liquid mode retains the captured RedGard finished-system rows. Sheet mode models one flat rectangle covered by one-piece KERDI strips with at least 2 in side laps and confirmed roll dimensions. No end splices, banded butt joints, corners, penetrations, accessories or waterproofing design are included.
- Coverage is approximate; transitions, reinforcing accessories, preparation and performance are not calculated.