Density and viscosity are independent

Mercury is 13× denser than water yet flows just as easily; honey is barely denser than water yet far more viscous.

Why it works

Density is about mass per volume; viscosity is about internal friction resisting flow — a fluid can be heavy and thin, or light and thick, with no contradiction.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

Open Fluid Properties →

More in Mechanical

• The bolt-torque shortcutTightening torque ≈ 0.2 × preload × bolt diameter (dry steel).
• Torque mostly fights frictionRoughly 90% of tightening torque is lost to friction; only ~10% actually stretches the bolt.
• Deflection limits: span/250Keep beam deflection under span/250 generally, span/360 where people will notice a bouncy floor.
• Depth beats width, cubedDouble a beam’s depth and its deflection drops 8-fold. Double its width and it only halves.

Browse all 96 rules of thumb →