Knife Billet Size Calculator for Bladesmithing

How to Use This Calculator

1. Choose your unit system (inches/lb or mm/kg) — the whole page updates.
2. Select your method — Forging (volume-conserved reshaping) or Stock Removal (grinding away material).
3. Pick your starting stock shape — flat bar, round bar, or square bar.
4. Enter your finished knife dimensions — blade length, ricasso length, max width, spine thickness; then tang length, width and thickness. These are the dimensions of the finished knife, not the starting bar.
5. Enter your starting bar cross-section — the width and thickness (or diameter) of the bar you plan to order.
6. Set a waste/loss allowance — scale loss and trim for forging (5–10%), or profiling waste for stock removal (10–20%). Beginners: use the higher end.
7. Add a steel price (optional) to get a cost estimate.
8. Read the Required Bar Length, weight, and cost. Use "Print / Save PDF" to keep a record.

The Formulas Explained

Finished Knife Volume

The calculator uses a rectangular approximation of each section. This slightly overestimates volume (real knives taper), which actually builds in a useful safety margin. The blade volume uses a 70% fill factor to approximate the blade profile (accounting for the taper toward the point and the bevel geometry):

Required Billet Volume (with loss factor)

Required Bar Length

Weight

Steel density used: 7.85 g/cm³ (0.284 lb/in³), per AISI standard for high-carbon steel including common blade grades 1084, 1095, 5160, and W2. Source: AmesWeb — Density of Steel by Grade.

When & Why You Need This

Experienced bladesmiths often calculate starting billet size by mentally "squishing" the finished shape — but that approach fails for beginners and leads to either running short mid-forge or wasting expensive steel. This calculator replaces the guesswork, homemade spreadsheets, and clay-modeling workarounds that the knifemaking community has used for years.

For Forging

Forging is a volume-conservation process: steel is moved, not removed. The billet you start with must contain at least as many cubic inches as your finished blade — plus the steel lost to forge scale (iron oxide that flakes off during heating) and any hot-cut or trim waste. For an efficient smith with a gas forge, 5–8% loss is typical. Beginners using a coal forge with multiple re-heats may lose 15–20%.

For Stock Removal

In stock removal you need a bar at least as thick as your finished spine (since you cannot add material) and as wide as your widest point. You then grind away the bevel and profile. The calculator adjusts the required length accounting for profiling waste from the saw cut and grinding loss outside the knife outline.

Planning a Batch

Use the "Number of Knives" input to price out steel for a production run or class. The cost estimate tells you exactly how much bar stock to order so you don't come up short or over-buy on a large purchase.

Frequently Asked Questions

How do I calculate what size billet I need to forge a knife?

For forging, steel volume is conserved: the starting billet volume must equal the finished knife volume plus allowance for scale loss (typically 5–10%) and any trim/hot-cut waste. Calculate the finished volume from blade dimensions, add your loss factor, then divide by your bar's cross-section to get the required starting length. This calculator does all of that math for you.

How much steel is lost during forging vs stock removal?

Forging loses relatively little material — typically 3–8% to forge scale and hot-cut waste for an experienced smith, up to 20% for beginners. Stock removal loses significantly more: you must start with a bar at least as thick as the spine, then grind away the full bevel depth and all profile waste. For a full-flat-grind knife, 30–50% of the starting bar volume may be removed.

What is the density of blade-making steels like 1084 and 1095?

Common high-carbon blade steels — 1084, 1095, 5160, W2 — all have a density of approximately 7.85 g/cm³ (0.284 lb/in³) per AISI standards. Variation between grades is less than 0.5% and does not meaningfully affect planning calculations.

What starting stock thickness should I use for stock removal?

Your bar must be at least as thick as the finished spine thickness. Most bladesmiths order standard flat bar: 1/8″ (3.2 mm), 3/16″ (4.8 mm), 1/4″ (6.4 mm), or 5/16″ (8 mm). Choose the next standard size up from your desired finished spine. This calculator shows a warning if your bar thickness is less than your spine thickness.

Can I forge a knife from round bar stock?

Yes. Many bladesmiths, especially those working with drill rod or salvaged round bar, forge from round stock. This calculator supports round bar: enter your bar diameter and it computes the required length. The cross-section formula is π/4 × d².

How much safety margin should I add to my billet?

For forging: 10–20% beyond the theoretical volume for scale loss, hot-cut waste, and finish grinding. Beginners typically need 15–25%. For stock removal: 5–10% extra length for saw kerf and profiling waste. Use the waste allowance slider to set your own margin based on experience.

Common Mistakes

• Not accounting for the tang volume — the tang can be 15–30% of total steel. Many beginners plan only for the blade and run short before the tang is drawn out.
• Using finished dimensions as starting dimensions (stock removal) — you must start with bar that equals or exceeds the finished spine thickness and width. Starting too thin leaves no material for grinding.
• Underestimating scale loss — every heat creates scale. Long forge sessions or coal forges with excess air can increase scale loss to 15%+. When in doubt, add extra length.
• Ignoring hot-cut waste — cutting the billet from bar stock with a hot-cut or angle grinder removes material at the cut. Add 3–5 mm (1/8–3/16 in) per cut.
• Not planning for distal taper — forging a distal taper moves steel forward and can increase the effective blade length available, but also changes the final width distribution. This calculator assumes uniform cross-sections as a conservative planning estimate.