House rules

Normally, Size Modifier is based on the largest dimension of an object. However, in many cases knowing the smallest dimension of an object is important as well.

For every character, calculate Length SM, Width SM, and Height SM separately, without using the awkward modifiers and special cases given on Basic Set pp. 19 and 550. Assume that the typical 2-yd-tall human is at most 1 yd wide and 2 ft long, so he has Width SM −2 and Length SM −3 in addition to his original Height SM +0. (Combat Writ Large in Pyramid vol. 3 no. 77 gives length∶width∶height ratios for horizontal and slithering characters and objects.) Typically, a character exposes two of his three dimensions to any attack, and a swinging attack uses the larger exposed SM as a modifier to hit, while a thrusting attack uses the smaller exposed SM plus 2 (or the largest exposed SM—whichever is less). (For example, if Aubrey swings his warhammer at Blanche, he typically will swing it sideways at her, because she's a lot taller than she is wide. Likewise, if Charles leans out from a hidden trapdoor in the ceiling and swings his sword from above at the unsuspecting David, he typically will swing his sword from David's back to his front or from his front to his back, because David is significantly wider than he is long. Finally, if Egbert is preparing to stab Frederick with a spear while hidden under spells of invisibility and silence, he probably will find it easier to stab Frederick in the back, rather than in the side, because Frederick's random movements are much less likely to bring his broad back out of the line of the spear than to shift his narrower side away from danger.)

Whether an attack is swinging or thrusting is determined by the GM and may differ from the attack's damage formula. For example, bullets are thrusting, though they have no thr in their damage formula. The swung attack of a pick also counts as thrusting, since the damaging portion of a pick is quite small and not even remotely similar to a line segment.

(The point of this change is to remove the above-referenced awkward modifiers and handle the special cases better. If you want to attack a rope, a snake, or a tractor-trailer, swinging at it obviously should be a lot easier than thrusting at it.)

For certain other purposes, such as Regular spells (Magic p. 11) and ST-cost discounts (Basic Set p. 15), calculating a Volume SM is necessary. Volume SM is equal to Length SM + Width SM + Height SM + 5 for a typical human of volume 0.15 yd³ (see Vehicles p. 80). For characters or objects that are more spindly and less boxy than a human, it may be considerably lower than that value. For characters or objects that are more boxy and less spindly than a human, it cannot under any circumstances be higher than Length SM + Width SM + Height SM + 7 (for a perfect box). It increases by +2 for every ×10 increase in volume (0.15, 0.5, 1.5, 5, 15, 50, 150,…).

For still other purposes, calculating Area SMs (Front/Back, Left/Right, and Top/Underside, separate from each other) may be necessary. An Area SM is equal to the sum of the two Length SMs that define the area, plus 2. It starts at +0 for the front/back of a typical human (nominally 2 yd², though actually closer to two-thirds or even half of that value—see Low-Tech Armor Design in Pyramid vol. 3 no. 52). It increases by +3 for every ×10 increase in area (1, 2, 5, 10, 20, 50, 100,…).

The default Damage Table on Basic Set p. 16 is rather laughable, with totally-arbitrary breakpoints at ST 10, ST 30, and ST 100. Instead of using it, set average swing damage to 0.35 × ST and average thrust damage to 0.175 × ST—or, in terms of the dice plus adds format:

(For generating and extending this table in a spreadsheet program, dice = floor(ideal average ÷ 3.5), and adds = round(ideal average − dice × 3.5, 0). If you prefer to imitate the original table in (1) including both negative and positive adds and (2) always using at least one die, set dice to max(round(ideal average ÷ 3.5, 0), 1), but don't change the adds formula.)

Each muscle-powered weapon's default modifier to damage is converted to a modifier to effective ST: Each ±1 to swing damage becomes ±30 % to ST, and each ±1 to thrust damage becomes ±60 % to ST. (This is calculated from the baseline damage of thr = 1.75 or sw = 3.5 at ST 10, so that characters near ST 10 will see minimal change from the default system: 1 ÷ 1.75 ≈ 60 %, and 1 ÷ 3.5 ≈ 30 %. If you prefer, you can use the raw proportions of ±4/7 and ±2/7.)

For example: A character with ST 15 has unmodified thrust damage of 3 and unmodified swing damage of 1d + 2. If he takes a swing with a saber (normally sw − 1; see Basic Set p. 273), his modified swing damage is taken from the entry for ST (15 × 70 %), or ST 11, which is 1d. If he thrusts with the same saber (normally thr + 1), his modified thrust damage is taken from the entry for ST (15 × 160 %), or ST 24, which is 1d + 1.

Non-weapon sources of modifiers to damage, such as maneuvers, skills, or techniques—especially those that give a modifier per die of damage—may also use this conversion, at the GM's discretion. For example, instead of adding +1 or +2 per die of unarmed thrust damage for high Karate skill (see Basic Set p. 203), add +60 % or +120 % to effective ST.

If you find the 3d6 bell curve confusing and hard to memorize, 1d10 + 5 is a serviceable approximation—if not for actual play, then at least for mental estimation and back-of-the-envelope calculation. Alternatively, if you find 3d6 insufficiently normal, 7d4 − 7 and 35d2 − 42 are more faithful to Gauss.

Any of these three sets of dice can be used as a drop-in replacement for 3d6, with the obvious caveat that rules relating to the edges of the distribution must be adjusted. For example: Under the 1d10 + 5 distribution, no result covers less than 10 % of the distribution, so you may want to abandon the rules for critical success or failure on natural rolls of 3 or 18, or you may want to replace them with a system of confirming critical successes/failures with repeated rolls—e. g., if you roll a natural 1 and confirm it with two further rolls of 1–2 on a d10, that's a critical success (overall chance is 1/250, which approximates the 1/216 of 3 on 3d6). Under the 7d4 − 7 distribution, a roll of 4 or lower—not just 3 or 4—always is a critical success, while a roll of 18 or higher—not just 18—always is a critical failure. And, under the 35d2 − 42 distribution, you can decree that critical successes/failures occur only on successes/failures by 10 or more, which are almost always possible under this wide distribution: even a skill-18 character has a nonzero chance of rolling 28!

Also possibly of interest (e. g., in rolling on GURPS Space tables):

And:

This rule replaces the Multiple Injuries rule in Conditional Injury (in Pyramid vol. 3 no. 120). It was devised in response to complaints that Multiple Injuries is a bit too chunky.

A character's existing wound level will stack with an incoming wound, if neither is a scratch (i. e., each has Severity −5 or higher) and they are within two Severity levels of each other. For example, a wound level of Severity −4 can stack with an incoming wound of Severity −2, −3, −4, or −5 (but not −6).

If the wound and the wound level have the same Severity, the character's new wound level is equal to that Severity plus 2. If one wound's Severity is lower than the other's by one level or by two levels, the character's new wound level is equal to the higher Severity plus 1. The stacking happens automatically; do not roll vs. HT.

Gross effect and pain level are based on the Severity of the wound level after stacking. However, shock penalty is based on the Severity level of the incoming wound, without stacking.

(Background: 1 yd + 1 yd is 2 yd, or an increase of two levels on the Size and Speed/Range Table. 1 yd + 0.7 yd is rounded down to 1.5 yd, or an increase of one level on the Size and Speed/Range Table. 1 yd + 0.3 yd is rounded down to 1 yd, or no increase on the Size and Speed/Range Table.)

Instead of six-sided dice, use a deck of 48 playing cards: a French pack with the kings and jokers removed. For each d6, draw a single card. Treat the result as a number (A = 1, J = 11, and Q = 12); if it's above 6, subtract 6 from it:

If you want to avoid card-counting shenanigans, reshuffle the deck not when it is exhausted but when some milestone is passed: after half the cards have been drawn, after all the spades have been drawn, after all the aces and queens have been drawn, etc. Alternatively, combine several 48-card packs into a single deck (four packs plus two suits from a fifth pack, if you want to have exactly 216 cards).

A maneuver that grants a bonus or penalty to an attack or defense roll can be declared retroactively. However, it grants only half the usual benefit to the user (rounded down) if taken in this fashion.

For example: If you fail a melee attack roll, you can retroactively declare that it was a Telegraphic Attack (granting +1 to the target's defense for each +1 that you get to your attack, up to +2) or an All-Out Attack (Determined) (adding +2 to your attack roll). If you succeed at an attack roll, you can retroactively declare that it was a Deceptive Attack (imposing −1 on the target's defenses for each −4 that you take on your attack). If you fail a defense roll, you can sacrifice your next turn to an All-Out Defense maneuver (in order to get a +1 bonus to defense). And so on.

Underlying many parts of GURPS (most obviously the Size and Speed/Range Table on Basic Set p. 550 and the Size Modifier Table on id. p. 19) is a geometric progression based on the sixth root of ten (and stretched down to the twelfth root of ten in a few places): 1, 1.5, 2, 3, 5, 7, 10. However, the implementation of this progression is very imprecise. This imprecision presumably is intentional, but it still results in some problems. For example, many tables in the Spaceships series very awkwardly use a 2, 6, 20, 60 progression rather than 2, 7, 20, 70. The tables in Conditional Injury also look more than a little strange.

The obvious solution is to increase the progression's precision (by consistently rounding it to the nearest two significant figures)—not so much that it's hard to memorize, but just enough that rounding errors are significantly reduced.

In sum: 1, 1.5, 2.2, 3.2, 4.6, 6.8, 10.

For example, all Spaceships tables that normally use the 2, 6, 20, 60 progression should be changed to use 2.2, 6.8, 22, 68. Likewise, a Conditional Injury Robustness Threshold of 8 now requires HP ∈ (38, 56] rather than HP ∈ (36, 60].

If you happen to be a robot, you can round everything to six binary digits rather than to two decimal digits. (Note that length is 2^{(SM + 2)×5/9} yd rather than 10^{(SM + 2)/6} yd. This binary progression grows faster than the decimal progression by a margin of 0.13 % per 1 SM, or 2.4 % per 16 SM, which does not lead to any major problems.)

HP represent only structural integrity. Reduction of HP represents only physical destruction.

When your HP would take injury from a source other than physical destruction (exhaustion, disease, poison, heart attack, bleeding, etc.), instead subtract the injury from a pool of HT-based HP (perhaps called healthiness points and costing ). If your HT is reduced to zero, you suffer a heart attack (Basic Set p. 429) or a breakdown (id. p. 485).

When your HP takes injury from physical destruction, that injury is also subtracted from your Metabolism Points.

(See Power-Ups 9 p. 30.)

A building's partition factor (Low-Tech Companion 3 p. 34) is its total floor, wall, and roof area divided by its volume, in units of ft²/ft³. (If no artificial floor is constructed, then do not count the ground as part of the building's floor area.) If you have a detailed floor plan, you can calculate the partition factor, rather than having to guess it from the guidelines on Low-Tech Companion 3 p. 34 and High-Tech Buildings (in Pyramid vol. 3 iss. 96) p. 8.

The partition factor used to construct nonstructural partitions and facings may be lower than 0.25 ft²/ft³. (For example, the exterior facing of the warehouse on High-Tech Buildings pp. 9–10 has an effective partition factor of (2 × (35 ft + 80 ft) × 20 ft + 35 ft × 80 ft) ÷ (35 ft × 80 ft × 20 ft) = 0.13 ft²/ft³.) However, structural partitions and facings must have a partition factor of at least 0.25 ft²/ft³.

This works for wielding weapons in three or four hands. However, the GM may rule that the grip of a weapon is too short for that weapon to be wielded with more than a certain number of hands.

If you're used to playing action-RPG video games like Dark Souls or God of War, you may expect a gigantic enemy to be unable to attack as often as a smaller PC can.

Interpret Move as yards per second rather than yards per turn.

This is based on the Gravity Table on Template Toolkit 2: Races p. 16.

The length of a character's turn is equal to 5 seconds divided by that character's Basic Speed (rather than the flat 1 second prescribed on Basic Set p. 362). Interpret Move as yards per second rather than yards per turn.

During combat, the GM should keep on the table a list showing the next two or three turns for all participants (à la Gladius or Gungnir), to prevent players from becoming confused.