Quick Answer
To calculate the number of retaining wall blocks needed for a circular retaining wall, you need to know the diameter of the circle and the dimensions of the blocks. Here is the basic formula:
Number of blocks = Diameter of circle x Pi () x Number of courses / Block length
For example, if the diameter of the circle is 10 ft, using blocks that are 1 ft long by 1 ft high, with 5 courses:
Number of blocks = 10 x 3.14 x 5 / 1 = 157 blocks
The key steps are:
1. Measure the diameter of the circular area.
2. Determine the dimensions of the blocks – length, height.
3. Decide how many courses or rows of blocks high you want the wall.
4. Use the formula: Diameter x Pi x Number of Courses / Block Length.
5. Round up fractional blocks to the next whole number.
Measuring the Diameter
The starting point for calculating retaining wall blocks for a circle is to measure the diameter of the circular area.
The diameter is the distance across the center of the circle. You can measure this directly if working on open ground. Simply measure from one side of the circle to the other going through the middle.
For example, you may be building a circular planting bed that is 5 feet across. The diameter would then be 5 feet.
If building along a curve, such as a curved garden bed or pond edge, you will need to measure the curve and estimate an average diameter. Take several measurements along the length of the curve to get an approximate diameter.
Be sure to convert any measurements to the same units. It’s best to use feet or meters when doing the calculations.
Tips for Measuring Diameter
– Use a tape measure, laser measure or measuring wheel to measure across the center of the circle.
– For curves, take measurements at several points along the length and average them.
– Convert all measurements to feet or meters for calculations.
– Round the diameter measurement to the nearest whole number or tenth.
– Measure along the centerline where the wall will be built. Account for any setback.
– Double check measurements to be sure – an inaccurate diameter will throw off block calculations.
Block Dimensions
The next key piece of information needed is the size of the retaining wall blocks that will be used. Standard blocks are available in different dimensions. Common sizes are:
– 8 in x 8 in x 4 in
– 12 in x 12 in x 4 in
– 18 in x 18 in x 4 in
– 24 in x 24 in x 4 in
The main dimensions needed for the block calculations are the length and height. Length is horizontal length of one face of the block. Height is the vertical height.
Most retaining wall blocks have a constant height of 4 inches. The length can vary from 8 inches up to 24 inches or more for larger blocks.
For example, a typical home retaining wall may use 12 inch long by 4 inch high blocks. A larger landscape wall may use 18 inch long blocks.
Determining Block Dimensions
– Check manufacturer specs for exact block sizes. Don’t estimate.
– Use a tape measure to measure block samples if needed.
– Use the precise length and height. Include fractional inches.
– Input the length in inches or feet as needed for the formula.
– Convert block height to inches or feet to match the diameter units.
– The thickness of blocks is not needed for calculating quantity.
Number of Courses
In addition to the diameter and block sizes, you also need to decide how many courses or rows of blocks high the wall will be built.
Retaining walls usually have multiple courses stacked on top of each other. The number of courses determines the total height of the finished wall.
For shorter walls, such as a one-tier garden wall, 3-5 courses is typical. Medium height walls may have 5-8 courses. Tall landscape walls often have 8 or more courses.
Each course added will increase the total number of blocks needed. So decide on the desired height when doing your calculations.
The number of courses is also affected by wall design and soil conditions. More courses may be needed for walls supporting steep slopes or heavy loads. Consult an engineer for walls over 4 feet high.
Tips for Determining Courses
– Consider the total height needed – measure if replacing an existing wall.
– Follow building codes for maximum height based on unsupported length.
– Use fewer courses for shorter ornamental or garden walls.
– Use 6-8 courses for larger landscape walls up to 4 feet tall.
– Get engineering advice for walls over 4 feet high or tiered walls.
– Factor in buried courses below ground level as part of total count.
The Calculation Formula
With the key information gathered, you can now use the formula to calculate the number of blocks:
Number of Blocks = Diameter x Pi () x Number of Courses / Block Length
Let’s break this formula down step-by-step:
Diameter – The diameter of the circle measured in feet.
Pi () – The mathematical constant 3.14.
Number of Courses – How many rows of blocks high the wall will be.
Block Length – The horizontal length of one block in feet.
To do the calculation:
1. Take the diameter length in feet.
2. Multiply by 3.14.
3. Multiply by the number of courses.
4. Divide by the block length in feet.
5. Round up any fractional blocks to the next whole number.
The final number is the estimated quantity of blocks needed for your circular retaining wall.
Calculation Formula Tips
– Use at least 3 decimal points for Pi or use 3.14. Don’t round to 3.
– Keep units consistent – diameter and block length must be in feet.
– Do the multiplication steps before dividing to avoid rounding errors.
– Divide block length after multiplying diameter x Pi x courses.
– Always round up the final number of blocks to the nearest whole number.
Calculation Example
Let’s go through an example to see the retaining wall block calculation in action.
Say we want to build a circular planter that is 5 feet in diameter using 12 inch long standard retaining wall blocks. We want the planter wall to be 3 courses high.
Diameter = 5 feet
Pi = 3.14
Number of Courses = 3
Block Length = 12 inches / 1 foot
Plugging this into the formula:
Number of Blocks = 5 x 3.14 x 3 / 1 = 47 blocks
Since fractional blocks aren’t possible, we round up to the next whole number for a total of 47 blocks needed.
To validate the number, if we multiply 47 blocks x 1 foot long per block, that gives us 47 feet of wall which should go around a 5 foot diameter circle.
This example shows how the formula takes into account the circumference of the circle based on diameter and the number of courses to calculate total blocks required.
Key Tips for Circle Wall Calculations
Here are some key tips to keep in mind when calculating retaining wall blocks needed for a circular wall:
– Measure diameter accurately along centerline of wall – calculate Pi do the rest.
– Confirm block dimensions – don’t estimate length and height.
– Decide on number of courses needed for total wall height.
– Use the formula: Diameter x Pi x Courses / Block Length.
– Keep units consistent for diameter and block length.
– Round calculated blocks up to the next whole number.
– Validate estimate by multiplying blocks x block length to match diameter.
– Add 5-10% extra for waste, imperfect circles and partial blocks.
– For larger projects, do a detailed takeoff from scaled plans.
Estimating Partials and Waste
The block calculation formula provides a good estimate of the number of full blocks needed. However, some additional blocks will be needed for partial blocks and wasting.
Partial blocks are needed to finish courses and make turns around the circle. Since blocks need to interlock, partials will be required to complete the pattern. Expect to use 10-20% partial blocks depending on diameter.
Waste accounts for blocks that get damaged or unusable cuts. Estimate 5-10% waste for a typical project. Some broken blocks can be used for partials.
For small projects, add 5-10% onto the estimated blocks to account for partials and waste. For larger projects with more courses, add 10-15%.
You may end up with leftover full blocks if estimating conservatively. Partial blocks and pieces can often be used creatively in the project too. Any unused full blocks can be returned or saved for future projects.
Tips for Estimating Partials and Waste
– Add 5-10% for smaller projects with just a few courses.
– Add 10-15% for larger multi-course circular walls.
– Plan for more partials on tighter curves with smaller diameters.
– Factor waste for breakage, chipping, cutting errors.
– Save unused full blocks for returns or future projects.
– Get credit if reusing partial blocks within the project.
– Take advantage of manufacturer block return policies.
Using Excel to Calculate
For larger retaining wall projects, it can be helpful to use Excel or other spreadsheet software to do the block calculations.
Excel allows you to plug in different diameters and block sizes to quickly get estimates. Formulas make it easy to adjust courses and do partials/waste estimates.
Here are some tips for using Excel:
– Create columns for diameter, block sizes, courses, partials %, etc.
– Use the Pi() function to get accurate Pi values.
– Build the formula in one cell, then copy across to other diameter scenarios.
– Add columns for partial blocks and waste factors.
– Make different sheets for different block sizes or layouts.
– Use color coding and formatting for clarity.
– Add tables or charts to visualize and compare options.
Sample Excel Retaining Wall Calculator
| Diameter | Block Length | Courses | Block Calculation | Blocks Needed | Partial Blocks | Waste | Total Blocks |
|---|---|---|---|---|---|---|---|
| 10 | 1 | 5 | =B3*PI()*C3/B2 | =ROUNDUP(D3,0) | =E3*10% | =E3*5% | =E3+F3+G3 |
This sample Excel table shows the retaining wall block calculation and factors in partials and waste to get a total block quantity estimate. The formulas can be copied down for different scenarios.
Using Plans for Takeoffs
For large scale landscape retaining wall projects, the most accurate way to determine blocks needed is by doing a takeoff from the project plans.
The plans will show the exact length of each segment of the wall at full scale. You can get precise lineal footage measurements directly off the plans.
Then multiply the lineal footage by the number of courses and blocks per linear foot to get total blocks.
This takeoff method accounts for curves, corners, steps and other details in the wall layout. Software programs can help automate and speed up the quantification process.
Having the total takeoff quantity from plans makes procurement and planning easier and can serve as a baseline for cost estimating. Always add waste and partials factors as well.
Tips for Takeoffs from Plans
– Use CAD or other software to scale measurements off plans.
– Get lineal footage for straight sections and arcs.
– Multiply lineal footage by number of courses.
– Divide by block length to get blocks per section.
– Add up blocks for all sections.
– Factor 10-15% for partials and waste even with takeoffs.
– Compare takeoff total to formula estimate as a sanity check.
Ordering Blocks for Delivery
Once you’ve calculated the number of retaining wall blocks required for your project, the next step is ordering them for delivery.
Depending on the quantity needed, you may be able to pick up blocks yourself from a landscape supply store. For larger quantities, bulk delivery from a retailer or wholesaler is more practical.
When ordering:
– Order 5-10% extra to allow for damages, waste, partials needed.
– Check lead times – some block styles may require weeks for order fulfillment.
– Inquire about job site delivery options – boom truck vs pallet.
– Clarify unloading expectations – blocks can weigh 2,000-3,000 lbs per pallet.
– Time the delivery close to when you will start construction.
– Have a space prepared to unload, stage and store pallets.
– Inspect all blocks upon delivery for damage and verify quantities.
Ordering the properly calculated amount of retaining wall blocks is key for an efficient wall building process. Do the math ahead of time to determine how many you need. Then work with suppliers to get the blocks when and where you need them.
Conclusion
Calculating the number of retaining wall blocks needed for a circle involves measuring the diameter, knowing the block dimensions, deciding on courses, and using the formula:
Diameter x Pi x Courses / Block Length
Measure diameter accurately, confirm block sizes, determine needed courses, plug into formula, and add for partials and waste.
For small projects, the formula provides a good estimate. For larger walls, do a takeoff from plans for precise quantities.
Ordering the calculated number of blocks assures you’ll have enough on hand to complete the circular retaining wall project.
