Count posts from the layout, not by dividing the entire perimeter as one line. Break the railing into straight centreline sections, identify every terminal, shared corner, and opening boundary, then divide each uninterrupted run into equal spans that do not exceed the maximum spacing you selected from the applicable design or product instructions.
The deck-railing post calculator counts shared physical endpoints once and reports terminal, corner, opening, and intermediate posts separately. It estimates quantity; it does not select post size, attachment, or structurally adequate spacing.
Measure each run
- Draw a plan view of the deck edge.
- Give every straight section a label and measure its post-centre length.
- Mark where two sections share one physical corner post.
- Mark each gate or opening with its section, offset, and width.
- Obtain a maximum post spacing from the selected railing system, approved design, or qualified reviewer.
- Calculate each uninterrupted run separately.
- Review the calculated actual spacing and post categories before buying.
Worked example: two runs sharing a corner
Suppose section A is 12 feet and section B is 12 feet. They meet at one shared corner, and the chosen maximum post-centre spacing is 6 feet.
Each section divides into ceil(12 / 6) = 2 spans and therefore needs one intermediate post. The physical endpoints are the first terminal, the shared corner, and the last terminal.
The planning count is:
- 2 terminal posts.
- 1 shared-corner post.
- 2 intermediate posts.
- 5 posts total before any user-selected spares.
Counting each section independently as three posts would produce six and double-count the shared corner. Treating the 24-foot path as one straight line would hide the corner condition.
Post-count decision checklist
- Open end: count one terminal condition.
- Shared corner: connect both sections to the same endpoint so the physical post is counted once.
- Gate or opening: identify both boundaries; they interrupt the ordinary railing run.
- Long uninterrupted run: divide it into equal spans no longer than the selected maximum.
- Stair transition: keep it separately identified because stair geometry and connections are outside a simple horizontal post count.
- Extra stock: add only a deliberate spare quantity, not a hidden percentage.
Common mistakes
- Measuring the outside perimeter when the calculator expects post-centre section length.
- Subtracting an opening from the total but not placing its two boundary conditions.
- Counting a corner once for each adjoining section.
- Assuming an attractive equal spacing is structurally acceptable.
- Using one spacing value across products with different approved details.
- Treating a material count as an attachment or footing design.
For a broader railing takeoff, first create a labelled sketch with the deck-railing measurement guide, then use the deck-railing materials calculator.
For supported feet, inches, fractions, and metric notation, use the general measurement guide.
Limits and sources
Guard-post loading, member size, connectors, blocking, corrosion protection, and attachment details are outside this guide. The American Wood Council DCA 6 deck guide illustrates one prescriptive wood-deck context and states that jurisdictional approval and the controlling code apply. The selected railing manufacturer's installation documents also control product-specific post placement. The railing formula methodology documents how MaterialNest splits runs and deduplicates shared endpoints.
Reviewed: July 29, 2026. Sources were checked on that date. Send corrections to feedback@materialnest.com.