Best Free Engineering Calculators 2026 — CNC Speeds, Pipe Flow, Reynolds Number and Heat Transfer

Spindle speed (RPM) is calculated as cutting speed multiplied by 1000, divided by pi times tool diameter. Feed rate equals RPM times the number of flutes times chip load per tooth. The cutting speed depends on the workpiece material and tool material. The Speeds and Feeds Calculator handles these formulas and includes material presets for common combinations like aluminium with carbide tooling.

For a metric thread, the tap drill size equals the nominal diameter minus the pitch. For example, an M10x1.5 thread uses an 8.5mm drill (10 - 1.5). For imperial threads, the formula is nominal diameter minus (1 divided by threads per inch). The Tap Drill Calculator supports both systems and lets you adjust thread engagement percentage for different materials.

Reynolds number (Re) equals fluid density times velocity times pipe diameter, divided by dynamic viscosity. Below Re 2,300 the flow is laminar (smooth, predictable). Above Re 4,000 the flow is turbulent (chaotic, higher friction). Between 2,300 and 4,000 is the transitional zone. Reynolds number determines which friction factor equations to use for pressure drop calculations.

Use the Darcy-Weisbach equation: pressure drop equals the friction factor times pipe length divided by diameter, times fluid density times velocity squared divided by two. The friction factor depends on the Reynolds number and pipe roughness, typically found using the Moody chart or the Colebrook equation. The Pressure Drop Calculator handles all of this automatically including fittings and valves.

LMTD stands for Log Mean Temperature Difference. It is the driving temperature difference used in heat exchanger design with the equation Q = U x A x LMTD, where Q is heat duty, U is overall heat transfer coefficient, and A is surface area. LMTD accounts for the fact that the temperature difference between hot and cold fluids changes along the length of the exchanger. Counter-flow exchangers have a higher LMTD than parallel-flow for the same inlet and outlet temperatures.

The ideal gas law is PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the universal gas constant (8.314 J/mol/K), and T is absolute temperature in Kelvin. It accurately models gas behaviour at moderate pressures and temperatures. At very high pressures or low temperatures, real gas corrections (like the Van der Waals equation) are needed. The Ideal Gas Law Calculator solves for any missing variable and handles unit conversions.