Density Constants
Standard Bag Yields
*Approximate yield after water addition.
Water Ratio Tip
Always start with 3/4 of targeted water. Excess water significantly reduces structural PSI.
Adding a 10% waste factor is industry standard to account for spills, uneven forms, and consolidation.
Concrete Estimator – Tube Logic
Frequently Asked Questions
Muhammad Shahbaz Siddiqui
Founder, TheCalculatorsHub
How I used the tube estimator to buy the right number of bags for fence posts
I was setting 12 fence posts in 200 mm diameter tube forms, each 900 mm deep. Before driving to the hardware store, I wanted a firm bag count. Eyeballing it had cost me a wasted return trip on a previous project, so this time I used the calculator first.
The calculator returned 34 litres total across all 12 tubes, which translated to 2 bags of 20 kg fast-setting concrete mix per tube and 3 bags for the last two posts. I bought 26 bags total. The Portland Cement Association's guidance on post setting recommends filling within 2 hours of mixing to avoid strength loss, so having everything on-site before starting was critical. All 12 posts were set in a single afternoon and showed no lean after the first week of curing.