River Rock Calculator

How to use this river rock calculator

What this calculator does. It estimates the surface area of a rectangular bed, the uncompacted rock volume, an added wastage allowance, the corresponding bulk weight, and an estimated material cost. It is useful for garden borders, dry creek beds, pool surrounds, pathways, and drainage cover where a consistent average depth is appropriate. It does not determine structural suitability, drainage design, slope stability, local permitting, or the exact quantity a supplier will deliver.

When to use it. Use it before requesting supplier quotes, comparing bagged rock with bulk delivery, checking whether a vehicle can carry the order, or budgeting a landscape project. For irregular areas, divide the site into rectangles, calculate each part, and add the quantities.

How to calculate. The calculator opens with a complete demonstration: a 12 ft by 8 ft bed, 2.5 in deep, using pea gravel at 111.62 lb/ft³ with 5% wastage. The example is calculated immediately and its Excel workbook is ready to download. (1) Choose US customary or Metric; switching units converts the current entries. (2) Select Rock type, or choose Custom density and enter your supplier's value. (3) Replace Length, Width, and Depth with measured positive values. (4) Enter Wastage as a percentage, normally 5-10% for ordinary placement. (5) Optionally enter either price field. (6) Read the live results and the breakdown table, then use Download Excel to export the current calculation. Reset clears the demonstration and all results; Excel remains disabled until a complete valid project is entered again.

Input guide. Rock type is required and sets a typical dry bulk density. Density is required, must be greater than zero, and accepts a plain decimal such as 111.62 lb/ft³ or 1788 kg/m³; moisture and grading can change actual density. Length and Width are required positive dimensions, for example 12 ft and 8 ft. Depth is required and represents the average installed thickness, for example 2.5 in; confusing inches with feet is a common large error. Wastage is required from 0% through 100%; a larger percentage increases both order volume and weight. Price per unit mass and Price per unit volume are optional nonnegative U.S. dollar rates. When both are entered, the calculator shows the lower of the two independently calculated cost estimates so you can compare supplier pricing bases.

Output guide. Area is the exact rectangle area. Base volume is area multiplied by depth before contingency. Volume needed adds wastage and is the amount to order before supplier rounding. Weight needed multiplies that volume by density and is an estimate rather than a scale-ticket value. Estimated cost uses the available price input; zero means a price of zero was entered, while a dash means no price was supplied. The table repeats each formula step so you can audit the result.

Worked example. The startup bed area is 12 × 8 = 96 ft². A 2.5 in depth equals 2.5 ÷ 12 = 0.208333 ft, so base volume is 96 × 0.208333 = 20.00 ft³. Adding 5% gives 21.00 ft³. At 111.62 lb/ft³, estimated weight is 21 × 111.62 = 2,343.96 lb. At $7.50 per ft³, estimated material cost is $157.50.

Learn more. The NIST guide to SI volume units explains cubic measurement, while the USDA NRCS Engineering Field Handbook provides broader context for earthwork, drainage, and site planning.