Post Hole Concrete Calculator

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Post Hole Concrete Calculator

Planning a fence, deck, or mailbox post installation? Our Post Hole Concrete Calculator helps you accurately determine the amount of concrete mix you'll need. Over-buying leads to waste and extra cost, while under-buying means frustrating trips back to the store. This tool takes into account the number of posts, the dimensions of your holes, and even the size of your posts to give you a precise estimate, ensuring your project goes smoothly.

How to Use the Calculator:

  1. Number of Posts: Enter the total number of posts you plan to install.
  2. Hole Diameter (inches): Measure the diameter of the hole you will dig for each post.
  3. Hole Depth (inches): Measure the depth of the hole. Remember to dig below the frost line in your area if applicable.
  4. Post Diameter (inches): Enter the actual diameter of your post (e.g., a 4×4 post is typically 3.5 inches). If you're not using a post (e.g., just filling a hole), enter 0.
  5. Concrete Bag Size: Select the size of concrete mix bags you plan to purchase.
  6. Click "Calculate Concrete" to see your results.

Understanding Concrete for Post Holes:

Concrete provides stability and strength for posts, preventing movement due to wind, soil erosion, or frost heave. The amount of concrete required depends on the volume of the hole that needs to be filled, minus the volume occupied by the post itself. It's always a good idea to round up your bag count to ensure you have enough, as slight variations in hole digging or concrete mixing can affect the final volume.

Tips for Pouring Concrete:

  • Prepare the Hole: Ensure the hole is clean and free of loose soil. A layer of gravel at the bottom can help with drainage.
  • Position the Post: Use a level to ensure your post is perfectly plumb before pouring concrete. Bracing the post temporarily is often necessary.
  • Mix Correctly: Follow the manufacturer's instructions for mixing concrete. Too much water weakens the concrete, while too little makes it difficult to work with.
  • Pour in Layers: Pour concrete in layers, tamping each layer to remove air pockets.
  • Slope the Top: Create a slight dome or slope around the base of the post at ground level to shed water away from the post, preventing rot.
  • Cure Properly: Keep the concrete moist for several days after pouring to allow it to cure properly and achieve maximum strength.








50 lb (approx. 0.375 cu ft) 60 lb (approx. 0.45 cu ft) 80 lb (approx. 0.6 cu ft)

function calculateConcrete() { var numPosts = parseFloat(document.getElementById('numPosts').value); var holeDiameterInches = parseFloat(document.getElementById('holeDiameter').value); var holeDepthInches = parseFloat(document.getElementById('holeDepth').value); var postDiameterInches = parseFloat(document.getElementById('postDiameter').value); var bagYieldCubicFeet = parseFloat(document.getElementById('bagSize').value); var resultDiv = document.getElementById('result'); // Input validation if (isNaN(numPosts) || numPosts <= 0 || isNaN(holeDiameterInches) || holeDiameterInches <= 0 || isNaN(holeDepthInches) || holeDepthInches <= 0 || isNaN(postDiameterInches) || postDiameterInches = holeDiameterInches) { resultDiv.innerHTML = "Post diameter must be less than hole diameter."; return; } // Convert dimensions from inches to feet var holeRadiusFeet = (holeDiameterInches / 2) / 12; var holeDepthFeet = holeDepthInches / 12; var postRadiusFeet = (postDiameterInches / 2) / 12; // Calculate volume of the hole (cylinder) in cubic feet var holeVolumeCubicFeet = Math.PI * Math.pow(holeRadiusFeet, 2) * holeDepthFeet; // Calculate volume of the post (cylinder) in cubic feet // If postDiameterInches is 0, postVolumeCubicFeet will be 0, which is correct. var postVolumeCubicFeet = Math.PI * Math.pow(postRadiusFeet, 2) * holeDepthFeet; // Calculate net concrete volume per hole var netVolumePerHoleCubicFeet = holeVolumeCubicFeet – postVolumeCubicFeet; // Calculate total concrete volume needed for all posts var totalConcreteVolumeCubicFeet = netVolumePerHoleCubicFeet * numPosts; // Calculate number of bags needed, rounding up to the nearest whole bag var numBags = Math.ceil(totalConcreteVolumeCubicFeet / bagYieldCubicFeet); // Display results resultDiv.innerHTML = "

Calculation Results:

" + "Total Concrete Volume Needed: " + totalConcreteVolumeCubicFeet.toFixed(2) + " cubic feet" + "Number of Concrete Bags Needed: " + numBags + " bags"; }

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