Introduction
Hypothetical case study — illustrative example, not a real project. The best way to understand deck post estimating is to watch it done end to end on a realistic project. This walkthrough takes a hypothetical 16×20 ft (4.88×6.10 m) elevated deck from dimensions to a fully costed materials list — post layout, counts, footing sizes, concrete volume, and line-item costs that sum exactly. Follow along with the Deck Post Calculator to reproduce every number.
The Project Assumptions
Our hypothetical deck measures 16 ft out from the house by 20 ft along the house (4.88 m x 6.10 m). It is ledger-attached on the 20-ft side, elevated with beams 5 ft (1.52 m) above grade — so 6×6 posts are specified. Two beam lines run parallel to the house, each 20 ft (6.10 m) long, with posts spaced 8 ft (2.44 m) apart per the beam span tables. Footings are 18 in (45.7 cm) in diameter and 42 in (107 cm) deep to clear a 36-inch frost line. Guardrail runs along the three exposed sides. Soil is assumed to be undisturbed native soil with normal bearing capacity — soft fill would demand wider footings and rock would add excavation cost, both flagged as cost drivers in the cost guide. All prices are typical US planning ranges; your local numbers will differ.
Post Layout and Counts
Posts per beam = (20 / 8) + 1 = 2.5, rounded up to 3 intervals, giving 4 posts per beam at 0, 6.67, 13.33, and 20 ft (0, 2.03, 4.06, 6.10 m). Two beams: 4 x 2 = 8 structural posts, 6×6. Post length: 5 ft above grade (60 in / 152 cm) + 6 in (15 cm) embedment into the pier + 2 in (5 cm) seat allowance = 68 in (173 cm) minimum — buy 6×6 x 12 ft (3.66 m) posts and trim, since 8-ft stock is too short and cutting in place handles grade variation (see the pro tips guide).
Guardrail posts: exposed perimeter = 20 + 16 + 16 = 52 ft (15.85 m). 52 / 6 = 8.67, round up to 9 intervals, plus 4 corners = 13 guardrail posts (4×4 x 42 in, pressure-treated). The structural and railing counts are estimated together in the complete guide, which uses the same method on a smaller deck.
Footing Sizes and Concrete Volume
Eight structural footings at 18 in diameter x 42 in deep (45.7 x 107 cm). Per-footing volume = pi x 9^2 x 42 = 10,681 in^3 = 6.18 ft^3 (0.175 m^3). Eight footings = 49.5 ft^3 = 1.83 yd^3 (1.40 m^3). At 0.6 ft^3 per 80-lb bag, that is 82.5 bags — round up to 83 bags, though at this volume ready-mix delivery is worth pricing (the estimating guide covers the crossover). Metric check: 8 x 0.175 m^3 = 1.40 m^3, and 1.83 yd^3 x 0.7646 = 1.40 m^3 — the two unit systems agree.
Costed Materials List
| Line item | Qty | Unit price | Extended |
|---|---|---|---|
| 6×6 x 12 ft pressure-treated posts | 8 | $46.00 | $368.00 |
| 80-lb concrete mix bags | 83 | $6.40 | $531.20 |
| Standoff post bases (6×6) | 8 | $14.50 | $116.00 |
| Beam-to-post caps | 8 | $11.25 | $90.00 |
| Through-bolts and structural screws kit | 1 | $48.00 | $48.00 |
| Gravel base (0.5 cu yd / 0.38 m^3) | 1 | $35.00 | $35.00 |
| Total | $1,188.20 |
Verification: $368.00 + $531.20 = $899.20; + $116.00 = $1,015.20; + $90.00 = $1,105.20; + $48.00 = $1,153.20; + $35.00 = $1,188.20. This covers the structural post-and-footing package only — guardrail posts, beams, joists, decking, and labor are separate. Add 10% contingency ($118.82) for waste and price movement, bringing the planning budget to about $1,307. Keep supplier receipts with this table — the bag count backs up your pour log if the inspector asks about footing depth.
Lessons from the Walkthrough
Three patterns worth noting. First, concrete dominates: nearly 45% of this package is concrete, the line item most often underestimated — always compute pi x r^2 x h rather than guessing. Second, the 8-foot spacing decision drives everything: tightening to 6 ft would add a post per beam (10 posts, 10 footings, ~25% more concrete and hardware). Third, buying 12-ft posts for 68-inch needs looks wasteful but is correct — grade varies, posts are cut in place, and splicing is never allowed. Reproduce this entire estimate for your own dimensions with the formula derivations as your check.
A fourth lesson hides in what the table excludes. The 13 guardrail posts are a second lumber order of roughly $200-$350 plus rail connectors — on this hypothetical deck they would add about 25% to the post package, which is why the estimating guide insists on counting railing posts in the same pass. And the 10% contingency is not padding: lumber prices move, one footing always over-digs, and a single bad 6×6 gets rejected at the saw. Budget the contingency, spend what you must, and the project finishes on budget instead of on hope.
Related Guides
- Deck Post Complete Guide — the flagship overview with a smaller worked example.
- Deck Post Cost Guide — price ranges behind every line item above.
- Deck Post Estimating Guide — the takeoff method used in this walkthrough.
- Deck Post Formulas Explained — derivations for the post-count and volume math.
- DIY vs Hiring a Pro — what this package costs installed by a contractor.
Get Your Number
Hypothetical case study — illustrative example, not a real project. Your deck has different dimensions, a different frost line, and different local prices — but the method is identical. Enter your numbers in the Deck Post Calculator, build the same style of line-item table, verify the arithmetic sums exactly, and you will walk into the lumberyard (or the contractor bid review) with a budget you can defend.
Disclaimer: This is a hypothetical illustration with typical US planning prices, not a real project quote. Results are planning estimates only. Verify quantities and prices with your supplier and consult a licensed professional or your local building authority for structural or code-related decisions.
