How to measure a retaining wall &h ow many blocks do I need?
If you are thinking about adding a stepped barrier to hold back soil on your property, the first question usually feels simple. Then it starts to feel less simple. You look at the slope, the ground changes, the corners are not square, and suddenly a tape feels like the wrong tool. I have seen this a lot around Calgary yards, where grades shift faster than expected and frost plays its own role.
At Sungreen Landscaping Inc, we often meet homeowners who already tried a rough calculation on paper. It looks fine at first glance. Then we walk the site and notice small details. A curve here, a dip there. Those details change material counts fast. A few extra courses or an added return can turn a tidy sketch into a short shipment. That pause, the moment of doubt, is usually where good planning begins.
You want clear numbers before you commit. Length along the ground. Vertical rise from finished grade. The size of each concrete unit, including the lip or setback. These are plain facts, but they are easy to misread when the yard slopes toward the house or away from it. I think this is where people get frustrated. Not because the math is hard, but because the site refuses to behave.
This article is written for property owners who want control before calling a crew, or who simply want to understand what they are paying for. You will see practical ways to size up your project and arrive at a realistic material total. No sales talk. Just the same thought process we use every week on Calgary builds, adjusted so you can follow it at your own pace.
Getting Accurate Site Dimensions and Material Counts
Before any concrete pieces arrive, you need a clear picture of what sits on your lot. Walk the line where the structure will stand. Follow every bend. Curves add length fast, straight runs rarely stay straight. I usually tell people to lay a garden hose along the edge and step back. It sounds basic, but it shows the real footprint better than a sketch.
Height causes more confusion than length. You look at the slope and guess. That guess is often wrong. Check the rise from finished ground at the lowest point to finished ground at the highest point. Use a long level and a tape, moving in steps. Write each step down. Add them up. This avoids surprises once concrete courses stack up.
Turning dimensions into a unit count
Once length and rise are clear, you can work out how many concrete pieces sit in each row. Divide the total run by the face width of one unit. Round up. There is always a cut somewhere. Multiply that result by the number of vertical courses. That gives a base count. Add extra for corners, caps, and mistakes. They happen. I have seen projects stall over three missing pieces.
Weight matters more than people expect. Handling a single unit feels fine. Handling fifty is different. If you want a better sense of load and labour, this guide on how much does a retaining wall block weigh gives real figures that help plan delivery and lifting.
Planning beyond the visible face
What you see from the yard is only part of the build. Backfill depth, base thickness, and drainage space all affect layout. Taller structures often call for added support behind the face. If your design pushes past four feet, read this article on how to reinforce a retaining wall. It explains why extra steps behind the structure save trouble later.
You do not need perfect math to plan well. You need honest dimensions and a bit of caution. Add spare units. Leave room for error. That small buffer usually costs less than a second delivery truck pulling into your driveway.
Finding the True Length Along Straight Runs and Curved Lines
The first mistake people make is trusting a quick glance. A straight run across a yard looks simple, but small shifts in grade or angle stretch the distance. Walk the full line with your feet. Mark each end. Pull a tape tight along the ground, not through the air. Keep it low. That small habit avoids short counts later.
Curves change everything. A gentle arc adds more length than you expect. I have watched homeowners draw a smooth line on paper, then act surprised when the numbers climb. Use a flexible hose or rope and follow the exact path where the structure will sit. Lay it flat. Measure the hose in sections if needed. Add those sections together. It takes time. It pays off.
Straight sections and clean corners
Break long runs into smaller pieces. Measure each straight span between corners. Write them down as separate numbers. Corners steal space. Inside corners shorten the face. Outside corners extend it. If your plan includes steps or returns, count them as their own runs. This keeps the math honest.
If you plan to add caps later, include their overhang in your numbers. That extra inch or two per piece adds up over a long run. It feels minor until delivery day.
Curves, radius, and real-world layout
For tighter curves, spacing between concrete units widens on the back edge. The face still follows the arc, but the base needs room. Mark the centre point of the curve and check the radius at several spots. Curves drawn by eye rarely stay consistent.
If this starts to feel like too much, that is normal. Yard shapes around Calgary are rarely perfect. Sites with slopes, trees, or utilities complicate things fast. A site visit from a crew experienced with landscaping and retaining walls can confirm your numbers before materials are ordered. That check often saves a second delivery and a long weekend of adjustments.
Take your time here. Length errors ripple through the rest of the build. Getting this part right makes every next step calmer.
Finding Vertical Rise Using Ground Levels
Height causes more trouble than length. People look at a slope and trust their eyes. That usually leads to a short build. You want numbers taken from the ground as it sits now and from the surface you expect once the job is done. Anything else is guesswork.
Stand at the lowest point where the structure will begin. This is your baseline. From there, work uphill in small steps. A long level and a tape work well. I think this step feels slow, but it shows the true rise without fancy tools.
- Place the level on the ground at the low end.
- Raise the far end until the bubble centres.
- Check the vertical gap between ground and level.
- Write the number down.
- Move forward and repeat.
Add all those vertical gaps together. That total gives you the real rise across the site. Slopes in Calgary yards often look gentle, then surprise you once snow melts and soil settles. This step catches that.
Finished grade matters too. Patios, sod, or gravel change where the top line sits. If you plan to add fill later, include it now. If you plan to dig down for a base, subtract that depth. I have seen projects end up too low because this part was skipped.
- Account for base depth below grade.
- Include cap height if used.
- Check local rules for maximum exposed height.
You might find two different numbers, one from the current ground and one from the planned finish. Use the larger one. It feels cautious, but it avoids rebuilding later. Once you know the true rise, the rest of the planning starts to settle into place.
Working Out the Count of Concrete Units per Course
Once the full run is clear, the next step is figuring out how each horizontal course comes together. This part is mostly arithmetic, but small choices affect the result. Take the total run along the ground and compare it to the face width of one concrete unit. Manufacturers list this clearly. A common size is about 12 inches wide, though some systems differ.
Divide the full run by that face width. You will almost never land on a clean whole number. Round up. Always. A shortfall of one unit stops work fast, especially once mortar or base material is down.
| Total run length | Unit face width | Units per course |
|---|---|---|
| 20 feet | 12 inches | 20 units |
| 24 feet | 16 inches | 18 units |
| 30 feet | 12 inches | 30 units |
Corners change the count. Outside corners add length. Inside corners reduce it. Each corner usually calls for special units or extra cuts. I think this is where paper plans drift from reality. Walk the line again and mark every turn.
Curves also affect spacing. Units fan out slightly on the back edge. The face stays tight, but you may lose one or two pieces per course compared to a straight run. That loss shows up late if you ignore it early.
If you plan to follow a traditional brick style layout, spacing rules differ. This guide on how do you build a brick retaining wall explains bonding patterns that influence horizontal counts.
You should also allow for waste. Cuts, chips, or small mistakes happen. A safe buffer is five to ten percent per course. It feels cautious. It usually saves time and delivery fees.
Planning for Extra Units, Cuts, and Material Loss
Even with precise site data, you should always include a buffer for unforeseen adjustments. Units get chipped. Corners require special cuts. Sometimes spacing shifts slightly once the first course is in place. A small allowance prevents delays and reduces stress on installation day.
Start by calculating the base count from your total run and vertical courses. Then add five to ten percent to cover waste. That number usually absorbs minor mistakes and small pieces that cannot be reused. It may feel like overestimating, but it saves you a trip to the supplier later.
Corner and curve considerations
- Outside corners often need one extra piece per turn.
- Inside corners may reduce the number slightly, but special cuts will still be required.
- Curved sections can expand spacing on the back, meaning extra units per course.
Plan for caps and any decorative topping separately. Their height and width affect total count. A miscalculation here often surprises homeowners after the main structure is complete.
Material longevity and backup
Some concrete units handle exposure better than others. If you use smaller bags or units, check their durability. This guide on do concrete bag retaining walls last explains factors that affect lifespan and performance. It can influence how much extra you buy to avoid replacing units too soon.
Extra planning takes little time and reduces frustration. A few extra units today are cheaper than a mid-project stop for more delivery. Always think ahead, even if the calculations feel slightly cautious. It pays off once your project stands complete.
Questions and answers:
What is the best way to determine the length for a garden barrier along curves?
For curved sections, simple straight-line estimates rarely work. Lay a flexible rope or hose along the path you plan to follow and mark it in segments. Measure each segment with a tape and add them together. This gives a more realistic run length. Don’t forget to include slight extensions for corners or bends, as the total distance often ends up longer than it looks on paper.
How do I figure out the total vertical height for a soil-holding structure?
Start from the lowest point of your site and move to the highest. Use a long level and tape in small steps to capture the rise accurately. Write each measurement down and sum them for a total vertical distance. Include planned fill or base depth as well, because ignoring these can leave the finished surface either too low or too high. Even minor differences affect the number of layers you will need.
How should I calculate the number of concrete units for a single row?
Divide the total horizontal run by the width of a single unit. Always round up. Corners and curves require extra pieces or cuts. For example, a 20-foot straight section with 12-inch units will need about 20 pieces, but an outside corner could add one or two more. If you prefer brick-style patterns, spacing affects the count as well. Always allow a few extra units to cover small mistakes or minor damage.
Why is it necessary to account for extra pieces and waste?
Even with accurate measurements, some units will break, need cutting, or not fit perfectly. Adding five to ten percent extra per layer usually covers these losses. Curved runs and corners typically increase the required count slightly. Planning for a buffer avoids stopping work mid-project to order more. You can also check material lifespan and suitability by looking at guides such as do concrete bag retaining walls last, which helps decide how much reserve material to have on hand.
