How it works
The calculator divides the opening by the nominal block size, keeps only whole blocks, and multiplies columns by rows.
Columns = ⌊ Opening width ÷ Module width ⌋
Rows = ⌊ Opening height ÷ Module height ⌋
Blocks = ⌈ Columns × Rows × (1 + Spare ÷ 100) ⌉
Blocks per area = 1 ÷ (Module width × Module height)
Reinforced joints = ⌊ (Rows − 1) ÷ k ⌋, k = ⌊ Spacing ÷ Module height ⌋
Reinforcing = Reinforced joints × Panel width
Anchors = 2 × Reinforced joints
Mortar bags = ⌈ Columns × Rows ÷ Blocks per bag ⌉
The module is the nominal block size – the actual block plus one joint. For a custom block it is the actual width plus the joint by the actual height plus the joint. ⌊ ⌋ means rounding down to whole blocks that fit; ⌈ ⌉ means rounding up to whole blocks or bags.
Nominal and actual sizes
Pittsburgh Corning sells glass block by nominal size: an 8 × 8 in block is 7¾ in across and is laid with a ¼ in joint, so each block takes 8 in of the panel. Its data gives 225 blocks per 100 sq ft for 8 × 8 in block, which is 144 ÷ 64 = 2.25 per square foot. European blocks are usually given by actual size: Seves lists its 19 × 19 × 8 cm block at 25 per square meter, laid with at least a 1 cm joint – a 20 cm module.
Reinforcing and anchors
Pittsburgh Corning’s specification places panel reinforcing in the horizontal joints every 16 in on center at most, starting after the first course, running from end to end of the panel and lapped at least 6 in where two lengths meet. Where panel anchors are used instead of channels, they go in the same joints. The calculator counts one length of reinforcing per reinforced joint and an anchor at each jamb. Add the joints above and below any opening inside the panel, the laps, and head anchors if your detail uses them.
Mortar
The bag count uses Pittsburgh Corning’s chart of blocks per 50 lb bag of its white premixed glass block mortar, which differs for its 4 in thick Premiere block and 3 in thick Thinline block. For other mortars or blocks, use the coverage on the bag.
Worked example
Inputs
- Opening width
- 4 ft
- Opening height
- 6 ft
- Block size
- 8 × 8 in (nominal)
- Block thickness
- Premiere Series (4 in thick)
- Reinforcing every
- 16 in
- Spare blocks
- 0 %
Result
- Blocks across
- 6
- Courses (rows)
- 9
- Glass blocks
- 54
- Blocks per area
- 2.25 / sq ft
- Panel width of whole blocks
- 48 in (4 ft)
- Panel height of whole blocks
- 72 in (6 ft)
- Joints with panel reinforcing
- 4
- Panel reinforcing length
- 16 ft
- Panel anchors at the jambs
- 8
- 50 lb bags of glass block mortar
- 3
Common mistakes
- Measuring the opening and dividing by the actual block size. The nominal size already includes the ¼ in joint, so an 8 × 8 in block takes 8 in of opening, not 7¾ in.
- Planning to cut blocks to fit. Glass block is not cut; size the opening to whole blocks, or fill the difference with framing.
- Counting reinforcing in every joint – or in none. Pittsburgh Corning calls for it every 16 in o.c. maximum and in the joints right above and below any opening within the panel.
- Forgetting the expansion strips and sealant at the head and jambs. Mortar must not bridge them.
- Ordering mortar by the panel area without checking the bag. Glass block mortar coverage differs between 3 in and 4 in thick block.
Assumptions and limits
- A flat, rectangular panel in stack bond with every joint aligned. Curved walls, corners and finishing units are not counted.
- Whole blocks only. Columns and rows are rounded down to what fits in the opening.
- Nominal sizes follow Pittsburgh Corning’s GB-185 table, which gives 225 blocks per 100 sq ft for 8 × 8 in block, 300 for 6 × 8, 450 for 4 × 8, 400 for 6 × 6 and 100 for 12 × 12, with ¼ in joints.
- Reinforcing runs continuously along every reinforced horizontal joint, one panel width long. Laps of at least 6 in where lengths join, and reinforcing above and below openings inside the panel, come on top.
- Panel anchors are counted at both jambs of each reinforced joint (Mortar II, panel anchor construction). Head anchors, channels and fasteners are not counted.
- Mortar bags use Pittsburgh Corning’s blocks per 50 lb bag of white premixed glass block mortar, rounded up to whole bags.
- This is a material estimate. Panel size limits, wind load, framing and support are design questions for the block maker’s data, local code or a qualified professional.
Frequently asked questions
How many glass blocks are in a square foot?
For 8 × 8 in nominal block, Pittsburgh Corning’s data gives 225 blocks per 100 sq ft, or 2.25 per square foot. 6 × 6 in block is 4 per square foot, 6 × 8 in is 3, 4 × 8 in is 4.5 and 12 × 12 in is 1. Seves lists 25 of its 19 × 19 cm blocks per square meter.
Why does the panel come out smaller than my opening?
Because glass block is not cut, only whole blocks fit. If the opening is 32 in wide and the blocks are 6 in nominal, five blocks make a 30 in panel and 2 in are left over. Pittsburgh Corning’s guides size openings to whole blocks; the difference is taken up in the framing.
How much mortar do I need for glass block?
Pittsburgh Corning’s chart says a 50 lb bag of its white premixed glass block mortar sets about 26 of its 4 in thick 8 × 8 in blocks, or 32 of the 3 in thick ones, with ¼ in joints. Other sizes are in the chart; the calculator uses it for the bag count.
How often does glass block need reinforcing?
Pittsburgh Corning’s specification calls for panel reinforcing every 16 in on center maximum, starting after the first course, plus the joints above and below openings within the panel. With 8 in block that is every other joint. Follow the guide for the block you are buying.
Can I use this for a glass block shower wall?
For the block count, yes. Shower walls often use end blocks and curved finishing units, which this calculator does not count separately; count those from the maker’s layout and subtract them from the plain blocks.
Sources
- Glass Blocks and Accessories – Physical & Design Data (Sweets presentation of brochure GB-185) — Pittsburgh Corning (Sweets, Dodge Construction Network). Number of block per 100 sq ft panel for nominal sizes: 6 in 400, 8 in 225, 12 in 100, 4 × 8 in 450, 6 × 8 in 300 (based on 1/4 in exposed joints); Premiere panels about 20 lb/sq ft installed; panels are non-load bearing.
- Glass Block Products Specification Guidelines GB-185 – Section 04270 Glass Unit Masonry — Pittsburgh Corning (hosted by AECinfo). "Install panel reinforcing every 16 inches o.c. maximum (starting after the first course) in the horizontal mortar joints"; run continuously, lap not less than 6 in; panel anchors at jambs and heads "in the same joints (16 inches o.c. maximum starting after first course)"; 7 3/4 in actual face of an 8 in block, 1/4 in joint.
- Mortar Glass Block Installation System – Assembly Instructions (Mortar I and Mortar II) — Pittsburgh Corning (hosted by Lowe’s). Consistent 1/4 in joints; blocks per 50 lb bag of white premixed glass block mortar – Premiere (4 in): 4 × 8 34, 6 × 6 34, 6 × 8 30, 8 × 8 26, 12 × 12 18; Thinline (3 in): 42, 42, 36, 32, N/A; Mortar II panel limits Premiere 250 sq ft interior / 144 sq ft exterior, Thinline 150 / 85 (20 psf).
- Clear 1919/8 Wave glass block – product data — Seves Glassblock. Dimensions 19 × 19 × 8 cm, "N. Pieces/m² 25 pcs./m²", 10 pieces per box; "must be installed with minimum space of 1 cm" (metric example: 190 mm block, 10 mm joint).
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 6, 2026