Ever stood in the lumber aisle wondering exactly how many balusters to grab? Buy too few and you're making another trip. Buy too many and you've got a pile of extras collecting dust in the garage. This calculator takes the guesswork out of it.
Enter your railing measurements, and you'll know exactly how many balusters you need—down to the last one. The calculator accounts for your posts, your baluster size, and your desired spacing, so you can buy with confidence and get your project done right the first time.
Whether you're replacing a worn-out deck railing, building a new porch, or finally tackling those stairs that have needed attention for years, accurate numbers mean less frustration and more time actually building.
What Are Balusters, Anyway?
Balusters are the vertical pieces that fill the gaps between your railing posts. Some people call them spindles. Others call them pickets. Your neighbor might call them "those stick things." They're all talking about the same component.
Here's the quick breakdown of railing anatomy:
Component | What It Is | What It Does |
|---|---|---|
Balusters/Spindles | The thin vertical pieces | Create a safety barrier so nobody falls through |
Posts | The thick vertical supports bolted to your deck | Hold the whole railing system up |
Top Rail | The horizontal piece you grab | Connects posts and gives you something to lean on |
Bottom Rail | The horizontal piece near the deck surface | Keeps balusters locked in place |
Why does this matter for your calculation? Posts take up space. If you've got a 10-foot railing with two 4-inch posts, you don't actually have 10 feet of space for balusters—you've got about 9 feet and 4 inches. The calculator handles this math for you, but understanding it helps you double-check your measurements.
The 4-Inch Rule: Why Spacing Matters More Than You Think
Here's something that catches a lot of first-time deck builders off guard: baluster spacing isn't just about looks. It's code. And inspectors check it.
What's the Rule?
The gap between your balusters can't exceed 4 inches (about 10 cm). Not 4.5 inches. Not "close enough." Four inches.
Why So Specific?
Picture a 4-inch sphere—roughly the size of a young child's head. Building codes are written so that sphere can't pass through any opening in your railing. It sounds morbid, but this rule exists because children have gotten their heads stuck in railings, and the results can be tragic. The 4-inch rule prevents that.
Regional Variations
Where You're Building | Maximum Gap | Good to Know |
|---|---|---|
United States | 4" (10 cm) | IRC code, but local jurisdictions can be stricter |
Canada | 100 mm | National Building Code standard |
United Kingdom | 100 mm | Part K of Building Regulations |
Australia | 125 mm | Slightly more lenient than North America |
Real talk: Your local building department might have stricter rules than the national code. If you're pulling a permit (and you probably should for deck work), call them first. A five-minute phone call beats tearing out a railing that fails inspection.
How the Math Actually Works
You don't need to understand the formula to use the calculator, but knowing how it works helps you catch mistakes before they cost you money.
The Basic Formula
Balusters Needed = (Railing Length − Post Space) ÷ (Baluster Width + Gap)
Let's break that down with a real example.
Walking Through a Real Calculation
Say you're working with:
- Railing section: 300 cm long
- Posts: 2 posts, each 10 cm wide
- Balusters: 5 cm wide
- Spacing: 10 cm gaps (code-compliant)
Step 1: Figure out your usable space Total length minus what the posts take up: 300 cm − (2 × 10 cm) = 280 cm of actual baluster space
Step 2: Calculate space per baluster Each baluster needs room for itself plus the gap after it: 5 cm + 10 cm = 15 cm per baluster
Step 3: Divide and round up 280 cm ÷ 15 cm = 18.67
You can't install 0.67 of a baluster, so round up: 19 balusters
Why round up, not down? Rounding down means your end spacing gets wider—potentially wider than code allows. Rounding up keeps you safe and legal. Plus, having one extra baluster is way better than being one short when you're halfway through installation on a Saturday afternoon.
How to Use This Calculator
Step 1: Measure Your Railing Length Get the total length of your railing section in centimeters. Measure from end to end, including where your posts sit.
Step 2: Count Your Posts How many posts are in this section? A basic straight run typically has 2 (one at each end). Longer runs might have intermediate posts every 6-8 feet for structural support.
Step 3: Measure Your Post Width Grab your tape measure and check the actual width. Don't trust the label—a "4×4" post is actually about 9 cm wide (3.5 inches). Lumber sizes are nominal, not actual.
Step 4: Check Your Baluster Width Measure the baluster at its widest point. Square balusters are easy. For turned or decorative styles, measure the thickest section.
Step 5: Set Your Spacing How big do you want the gaps? Stay at 10 cm or under to meet code. Tighter spacing (like 7-8 cm) gives a more substantial look. Just know that tighter spacing means more balusters.
Step 6: Get Your Number The calculator does the rest. That's your baluster count for this section.
Measuring Like a Pro (Without the Pro Price Tag)
Bad measurements lead to bad results. Here's how to measure each component accurately:
Railing Length
For existing railings you're replacing: Measure from the inside face of one post to the inside face of the opposite post. Then add one post width. This gives you the total section length.
For new construction: Measure the span where the railing will go. If you haven't installed posts yet, measure the total distance and account for post width in your planning.
Pro tip: For decks with corners, measure each straight section separately. Don't try to calculate the whole perimeter at once—you'll confuse yourself and probably mess up the math.
Post Width
Here's where people get tripped up. Lumber is sold by nominal size, but the actual dimensions are smaller:
What the Label Says | What You Actually Get |
|---|---|
4×4 post | 3.5" × 3.5" (about 9 cm) |
6×6 post | 5.5" × 5.5" (about 14 cm) |
Don't guess. Grab your tape measure and check. If you're using the wrong number, your baluster count will be off.
Baluster Width
Standard square wooden balusters run about 1.5 inches (4 cm). Metal balusters are often thinner—3/4 inch to 1 inch. Turned wood balusters vary based on the design.
If you haven't bought balusters yet: Check the product specs before calculating. Different widths change your total count significantly.
Real Projects, Real Numbers
Abstract formulas are fine, but seeing actual scenarios helps you sanity-check your own calculations.
Project 1: Basic Deck Railing
The situation: You're adding a railing to one side of a 4-meter deck. Standard setup with a post at each end.
Input | Value |
|---|---|
Railing length | 400 cm |
Posts | 2 |
Post width | 10 cm |
Baluster width | 5 cm |
Spacing | 10 cm |
Result: 25 balusters
Buying recommendation: Grab 27-28 to have a couple spares.
Project 2: Wraparound Porch with Intermediate Posts
The situation: Your porch railing runs 8 meters with structural posts every 2 meters (required for code on longer spans).
Input | Value |
|---|---|
Railing length | 800 cm |
Posts | 5 (including ends and intermediates) |
Post width | 10 cm |
Baluster width | 4 cm |
Spacing | 8 cm |
Result: 62-63 balusters
Buying recommendation: Round up to 70. Longer projects have more room for error, and you'll want extras for future repairs.
Project 3: Stair Railing
The situation: Your deck stairs need a railing on one side. The horizontal run of the stairs is 200 cm.
Input | Value |
|---|---|
Railing length | 200 cm |
Posts | 2 |
Post width | 10 cm |
Baluster width | 5 cm |
Spacing | 10 cm |
Result: 12 balusters
Important note for stairs: Measure the horizontal distance, not the angled length along the handrail. Balusters are typically installed plumb (straight up and down), so the horizontal measurement gives you the right count.
Project 4: Full Deck Perimeter (Multiple Sections)
The situation: You're doing a complete deck with railings on three sides and a gap for stairs.
Don't try to calculate this as one giant number. Break it into sections:
Section | Length | Posts | Balusters Needed |
|---|---|---|---|
North side | 500 cm | 2 | 32 |
East side | 300 cm | 2 | 19 |
South side (with stair gap) | 350 cm | 3 | 21 |
Total | 72 balusters |
Buying recommendation: Get 80. You've got three different sections, which means three opportunities for measuring errors or installation mistakes.
Mistakes That Cost Time and Money (And How to Avoid Them)
Learn from other people's Saturday afternoons gone wrong:
Mistake
Your railing length isn't all usable space. Two 4-inch posts eat up 8 inches of your railing. On a short section, that's the difference between needing 10 balusters and needing 12.
Fix: The calculator handles this automatically, but double-check your post count and widths.
Mistake
"I've got 4×4 posts, so I entered 4 inches."
Nope. Your 4×4 is actually 3.5 inches. This error throws off your calculation, and the problem compounds with multiple posts.
Fix: Measure your actual lumber. Every time.
Mistake
"Code says 4 inches max, so I'll space at exactly 4 inches."
Here's the problem: if your spacing is exactly 4 inches and your measurement is off by even a quarter inch, you fail inspection.
Fix: Target 3.5-3.75 inches (9-9.5 cm) to give yourself a margin of error. You'll sleep better.
Mistake
Balusters crack. They split when you're drilling. They have knots in bad places. One falls off the deck into the bushes and you can't find it.
Fix: Buy 10% more than you need. For a project needing 50 balusters, grab 55.
Mistake
Your deck has four sides, but they're not all the same length, and they don't all have the same number of posts. Calculating an "average" and multiplying doesn't work.
Fix: Calculate each section individually, then add them up.
Tips From People Who've Done This Before
Start from the center, not the end. Install your first baluster at the center of each section, then work outward in both directions. This ensures any small spacing variations end up at the posts (where they're less noticeable) rather than in the middle of your railing.
Make a spacer jig. Cut a scrap piece of wood to your exact spacing width. Use it between every baluster as you install. Eyeballing spacing leads to uneven results that you'll notice every time you look at your deck.
Check for plumb every few balusters. It's easy for things to drift as you go. A level check every 4-5 balusters catches problems before you've installed a whole section crooked.
Pre-drill pilot holes. Especially for hardwoods and near the ends of balusters. Screwing directly into the end grain without a pilot hole is asking for splits.
Keep your receipt and note the product details. Write down the exact baluster style, size, and where you bought them. Two years from now when you need to replace one that got damaged, you'll thank yourself.
Do a dry fit first. Lay out your balusters without fastening them to check spacing and appearance. Adjusting dry-fit balusters takes seconds. Adjusting installed balusters takes hours.
This calculator provides estimates based on your measurements. Always verify calculations manually for critical projects, and confirm all local building code requirements before purchasing materials or beginning construction. When in doubt, consult a licensed contractor or your local building department.