Foot pounds-force second to BTU/hour

James Watt calculated that a mill horse could turn a mill wheel 144 times per hour, doing 32,572 ft·lbf of work per minute — he rounded up to 33,000 ft·lbf/min (550 ft·lbf/s) for marketing purposes. He wanted to sell steam engines by comparing them to horses, so he likely overestimated the horse to make his engines look like better value. A real horse sustains closer to 350–500 ft·lbf/s, so Watt's "1 HP" is actually more than one horse.

Power (ft·lbf/s) = Torque (ft·lbf) × RPM × 2π / 60. This is the workhorse formula (pun intended) of US mechanical engineering. For example, an engine producing 200 ft·lbf of torque at 3,000 RPM delivers 200 × 3,000 × 6.2832 / 60 = 62,832 ft·lbf/s ≈ 114 hp. The formula works because angular velocity in rad/s times torque in ft·lbf gives power directly in ft·lbf/s.

Pushing with 1 pound of force at 1 foot per second — roughly the effort of slowly sliding a light book across a table against friction. Lifting a 1-pound weight 1 foot in 1 second. Turning a doorknob with a very light touch. It's about 1.36 watts — enough to dimly light an LED. In human terms, it's almost effortless: casual walking produces about 50–80 ft·lbf/s of mechanical power, and you don't even notice.

Yes, particularly in ballistics (muzzle energy rates), mechanical testing (dynamometer output), agricultural machinery specs, and industrial equipment designed for the US market. However, even in the US, many engineering firms are switching to SI units for international compatibility. The automotive industry increasingly quotes power in both hp and kW. Aerospace has been mostly metric since the 1990s. Ft·lbf/s survives mainly in traditional mechanical and manufacturing industries.

Bullets are rated in ft·lbf of muzzle energy (not per second), but the power of a firearm is the muzzle energy divided by barrel time. A .308 rifle bullet exits with about 2,600 ft·lbf of energy over a barrel transit time of ~0.001 seconds, meaning the instantaneous power is roughly 2,600,000 ft·lbf/s (about 3,500 hp). That's why rifle recoil feels punchy — for a millisecond, you're absorbing the reaction force of a truck engine.

The classic rule: 20 BTU/h per square foot. A 300 sq ft bedroom needs about 6,000 BTU/h; a 500 sq ft living room about 10,000 BTU/h. But this varies wildly with sun exposure (+10% for south-facing), ceiling height, insulation quality, number of occupants (+600 BTU per person), and climate zone. A room above a pizza oven in Phoenix needs more than a basement in Seattle. When in doubt, oversize slightly — an undersized unit runs constantly and never reaches setpoint.

Undersizing is obvious — the unit runs constantly and never reaches the thermostat setpoint on hot days. But oversizing is worse in subtle ways. An oversized AC cools the air quickly then shuts off before removing enough humidity, leaving you with a clammy 72°F house. The short cycles also wear the compressor faster (startup is the hardest moment) and waste energy. A 1-ton oversize in a humid climate like Florida can raise indoor humidity from a comfortable 45% to a muggy 60%. Proper Manual J load calculations matter more than most homeowners realize.

Exactly 12,000 BTU/h. One ton of AC is the cooling effect of melting one short ton (2,000 lbs) of ice over 24 hours. The ice absorbs 288,000 BTU of heat as it melts (2,000 lbs × 144 BTU/lb latent heat), divided by 24 hours = 12,000 BTU/h. Residential systems run 1.5–5 tons; commercial buildings 10–500 tons. The "ton" unit persists because HVAC contractors think in tons — "that house needs a 3-ton unit" is faster than "that house needs 10.5 kW of cooling."

Modern units achieve 12–25 BTU/h per watt of electricity (SEER 12–25). A SEER 20 unit removes 20 BTU/h of heat for every watt consumed — effectively a 3:1 heat pump ratio. That 12,000 BTU/h window unit draws 500–1,000 W of electricity depending on efficiency. The best mini-splits achieve SEER 30+, removing 30 BTU/h per watt, making them cheaper to run than resistive electric heaters even in heating mode.

A gas furnace's BTU/h rating is its thermal output after combustion efficiency losses (typically 80–96% of fuel input). A heat pump's BTU/h rating is the heat delivered including energy moved from outside — at COP 3, a heat pump delivering 36,000 BTU/h uses only 12,000 BTU/h worth of electricity. This makes direct BTU/h comparisons misleading: a 60,000 BTU/h furnace and a 60,000 BTU/h heat pump deliver the same heat, but the heat pump uses one-third the energy.