Preparation checklist: make separate measurable schedules
- Sketch the deck perimeter and number every railing opening, post, gate/stair interruption and non-railed edge. Mark the inside faces of posts; do not begin with an outside-to-outside railing dimension.
- For each railing product, record the exact compatible baluster, rail, post, connector and fastener system, document revision, stated dimensions and the project's supplied maximum gap. A target gap is an input, not a compliance finding.
- Make a separate lumber schedule: purpose/location, supplier's nominal dimensions, supplied piece count, actual dimensions if needed for a volume check, stock length, grade/species/treatment as specified, allowance policy and quote basis.
- Keep deck-board coverage, post-hole concrete, finish coverage and the deck budget as separate scopes. Capture the selected product SKU, availability, delivery, returns, tax and quote date before combining any prices.
- Obtain approved structural and railing details before purchasing posts, beams, joists, rails or connections. This guide intentionally does not size structural members.
What and how to measure before opening a calculator
Measure each baluster run between the inside faces of its posts, in one unit, and retain every run separately. Measure the selected baluster's supplied width and enter a project-supplied maximum clear gap. A post ends one run and starts another, so its two end gaps must not be pooled into a longer imaginary opening. Use the Baluster Spacing Calculator to solve those independent clear runs for the smallest whole baluster count and equal end/intermediate gaps.
For a lumber line, copy the supplier's nominal thickness, nominal width, nominal length and supplied piece count. Record actual physical dimensions separately only when you need the solid-volume check; they do not replace nominal dimensions in a board-foot tally. Use the Board Foot & Lumber Cost Calculator to keep nominal board feet, actual volume, an entered allowance and one local price basis visible.
The other deck worksheets remain inputs with different meanings: the Decking Calculator estimates board coverage/order, the Post Hole Concrete Calculator estimates entered hole geometry, the Fence & Deck Stain Calculator estimates finish coverage, and the Deck Building Cost Calculator keeps supplied quantities and quote scopes in a budget. None transfers quantities automatically, selects the same product system, or designs structure. For a related structural planning boundary, see the live Stair Building Basics guide.
C19 worked calculation: one clear run, equal gaps and a baluster order
Illustrative C19/UAT inputs only: one 2.00 m clear run, 0.050 m baluster width and a 0.100 m maximum target gap. These reproduce the independent fixture and are not a product or code recommendation. The calculator uses n = ceil((L − M) ÷ (w + M)), then g = (L − n × w) ÷ (n + 1).
| Stage | Calculation | Result |
|---|---|---|
| Balusters | ceil((2.00 m − 0.100 m) ÷ (0.050 m + 0.100 m)) | 13 balusters |
| Equal gaps | 13 balusters + 1 end/intermediate gap | 14 gaps |
| Equal clear gap | (2.00 m − 13 × 0.050 m) ÷ 14 | 0.096428571 m = 96.43 mm |
The order from this single run is 13 balusters. Repeat the calculation for every post-bounded opening and add the resulting whole counts. Do not divide a combined perimeter by a spacing target: it hides end gaps, posts and openings. Posts, rails, connectors, fasteners, cuts, spares and labour are separate purchase lines unless a selected system's documented schedule says otherwise.
C20 worked calculation: nominal board feet, actual volume and price basis
Illustrative C20/UAT inputs only: five supplied boards with nominal purchase dimensions 2 in × 6 in × 10 ft, actual physical dimensions 1.5 in × 5.5 in × 10 ft, 0% allowance, and an illustrative local rate of USD 3.00 per board foot. These values explain the worksheet; they are not a lumber-size, allowance or price recommendation.
| Stage | Calculation | Result |
|---|---|---|
| Nominal board feet per piece | 2 in × 6 in × 10 ft ÷ 12 | 10 bf/piece |
| Net and ordered board feet | 5 pieces × 10 bf/piece × (1 + 0 ÷ 100) | 50 bf |
| Actual solid volume check | 5 × 1.5 in × 5.5 in × 10 ft | 4,950 in³ = 0.081116 m³ |
| Illustrative extension | 50 ordered bf × USD 3.00/bf | USD 150.00 |
The independent decimal-rate C20 fixture uses an illustrative USD 3.005/bf instead: 50 bf × USD 3.005/bf = USD 150.25 after final-line currency rounding. That is a pricing-precision check, while the UAT's USD 3.00/bf example above expects USD 150.00. A board foot is a nominal purchasing tally; 50 bf is not a claim that the boards occupy 50 cubic feet or that their actual volume is 0.081116 m³. Conversely, the actual-volume check does not change a supplier's board-foot price basis. If the supplier quotes per piece, price the five supplied pieces at that basis instead; do not apply both per-piece and per-board-foot prices. The calculator's allowance expands ordered board feet but deliberately does not invent extra whole boards, stock lengths or bundle quantities.
Changed-input comparison: a smaller target gap changes the baluster order
Keep the C19 clear run and baluster width above, but change only the maximum target gap from 0.100 m to 0.090 m. The new count is ceil((2.00 m − 0.090 m) ÷ (0.050 m + 0.090 m)) = 14 balusters. That makes 15 equal gaps; each is (2.00 m − 14 × 0.050 m) ÷ 15 = 0.086666667 m = 86.67 mm. This matches the C19 UAT changed-input result. The one tighter input adds one baluster and one gap; it says nothing about the required post, rail, structural or fastener design.
Common ordering mistakes and practical purchase or estimate checklist
- Measuring across posts: outside-to-outside length is not a clear baluster opening. Measure each inside-face-to-inside-face run and solve it independently.
- Calling a target gap compliance: a calculator can keep an entered mathematical target; it cannot approve a railing system, connection, load, local rule or installation.
- Turning board feet into volume: nominal board feet and actual physical volume are different measures. Keep both labels and never price an actual-volume result as board feet without the supplier's stated basis.
- Inventing stock from allowance: board-foot allowance does not choose stock length or create pieces. Obtain sellable lengths, bundles and a cut policy from the supplier before ordering.
- Combining deck scopes: do not add decking boards, post-hole concrete, stain coverage, balusters and lumber lines merely because they are all “deck materials.” Preserve each source, unit, purchase rule and quote basis.
- Purchase / estimate check: attach labelled deck and railing sketches; clear-run schedule; selected compatible railing and lumber SKU/specification; nominal/actual dimensions; counted pieces; calculated balusters and gaps; supplier stock/pack rules; local quote currency/date; delivery, returns, tax and lead time; separate exclusions; and final structural/compliance approval.
Sources, scope, and limitations
Deckorators' Classic Baluster installation instructions (accessed 14 October 2026) are primary product instructions to check for a compatible selected railing system. They do not create a universal clear gap, post spacing, fastening pattern, load design or compliance result for the illustrative values below.
US Forest Service board-foot information (accessed 14 October 2026) supports treating a board foot as a conventional lumber tally. It does not select nominal or actual dimensions, grade, species, treatment, stock length, price, waste or structural use.
Scope limit: the synthetic values below reproduce the C19/C20 calculator and UAT checks; they are not spacing, lumber, cost, fastening or structural recommendations. Current selected-system instructions, local requirements, drawings, supplier confirmation and the responsible designer/installer control the work.