Size Modifier or SM represents an object or character's general size. The standard and most common Size Modifier is 0, which includes most adult humans, from ~4'4" to ~7'9". From -3 on this table matches up with the Size and Speed/Range Table.

Simply put, Size Modifier affects the difficulty of hitting a target.

"Size Modifier and Reach" discusses a bonus to grapple those of lower SM.[1] Unclear if this stacks with B19.

Important note: SM is totally different from Realm Size Value as the later is based on area not longest dimension.

Size Modifier Table

DouglasCole related in Extended Speed/Range Table that the formulas used are

"Speed/Range formula: 2 - 6*log(Distance in Yards)
Size formula: 6*log(length in yards) - 2
Round to nearest integer."

This overrules the basic set which states "If size falls between two values, base SM on the next-highest size." and "If the range falls between two values, use the higher; e.g., treat 8 yards as 10 yards"[2] This was likely because someone realized the original rule resulted in Murphy Rules level of silliness such as 9' being +1 but 9' 1" to 15' being +2.[note 1]

Note: 1 mile = 1760 yd; 1 km = 1093.61 yd

Reach Modifier only applies to the upper number of the reach range and only in relation to the SM of the opponent. So a SM+3 against SM+2 only gets a +1 to close combat only.

Tip on using the table

Use the fact log(xy) = log(x) + log(y) to find values not on the table without a calculator. (The formula becomes 6*log(x) + 6*log(6) - 2)

For example, 1 mile = 1760 yd. That is 1.7 x 10^3. 1.76 is -1 and the 3 means to use +18 (6*3) for Size resulting in a +17 for Size.

Earth is 7,917.5 mi or 13,934,800 yd which becomes 1.39348320 x 10^7. 1.39348 is -1 and 7 is 42 (7x6). Earth's Size modifier is +41

If you really want to figure the transition points the formula is ((Size + 0.5) + 2)/6 = log(length in yards) or log(y) = (x + 2)/6; just paste this equation into symbolab and plug in a Size (x)

Longest Dimension Size
0.05 yard (1.8”) -10 None Small beetle, all human sized rings
0.07 yard (2.5”) -9 None
0.1 yard (3.5”) -8 None
0.15 yard (5”) -7 None
0.2 yard (7”) -6 None Rats, Smaller human sized wands (9", -5.6)
0.3 yard (10”) -5 None
0.5 yard (18”) -4 None Larger human sized wands (14", -4.46)
0.7 yard (2’) -3 None
1 yard (3’) -2 None House cat
1.5 yards (4.5’) -1 None Average dog
2 yards (6’) 0 None Most Adult Humans, smaller staffs (6')
3 yards (9’) +1 +1 to C only Smallish giants, larger staffs (9')
5 yards (15’) +2 +1 Giants (Yrth);
7 yards (21’) +3 +2 Elephants (18' - 21'); Large Fantasy Folk Giants (21' to 24')
10 yards (30’) +4 +3 Large Fantasy Folk Giants (25' to 36')
15 yards (45’) +5 +5 Large Fantasy Folk Giants (37' to 45')
20 yards (60’) +6 +7
30 yards (90’) +7 +10 Larger whales; Blue Whale (98'); Argentinosaurus (110');
50 yards (150’) +8 +15 Godzilla, 1954 film (50 m/164')
70 yards (210’) +9 +20
100 yards (300’) +10 +30
150 yards (450’) 1/4 mi +11 Off chart Godzilla,1956 film (122 m/400') Empire State Building (424')
200 yd +12 Off chart
300 yd +13 Off chart Eiffel Tower (984'); Godzilla, 2017 film (300 m/984')
500 yd +14 Off chart
700 yards (2100') 2/5 mi +15 Off chart
1,000 yard; 0.568 mi +16 Off chart Burj Khalifa (906 yds)
1,500 yard; 0.853 mi +17 Off chart
2,000 yard; 1.136 mi +18 Off chart Largest starships in Spaceships
3,000 yard; 1.704 mi +19 Off chart
4,500 yard; 2.566 mi +20 Off chart
7,000 yard; 3.977 mi +21 Off chart
10,000 yard; 5.682 mi +22 Off chart Deepest part of Mariana Trench (6.825 mi), Eternal Crusader, Warhammer 40k (10 km)
10 mi (1,7600 x 10^4 yds) +23 Off chart Universe Mass Conveyor, Warhammer 40k (12 km)
15 mi (2.6400 x 10^4 yds) +24 Off chart Executor (Star Wars)
20 mi (3.5200 x 10^4 yds) +25 Off chart Independence Day's city destroyer (24.1 km)
200 mi (3.52000 x 10^5 yds) +31 Off chart
2,000 mi (3.520000 x 10^6 yds) +37 Off chart
7,917.5 mi (1.3934800 x 10^7 yds) +41 Off chart Diameter of Earth or Mondas (Doctor Who), Actual value ~40.8645
20,000 mi (35200000 yds) +43 Off chart
127530 km
(1.39468504 x 10^8 yds)
+47 Off chart Installation 00 (Halo), actual value ~46.867
86,881 mi
(1,52910560 x 10^8 yds)
+47 Off chart Diameter of Jupiter, actual value ~47.10
200,000 mi
(3.52000000 x 10^8 yds)
+49 Off chart
1,392,700 km
(1.523075241 x 10^9 yds)
+53 Off chart Sol, actual value ~53.096
200,000,000 km
(2.18722659668 x 10^11 yds)
+66 Off chart Dyson sphere ("Relics" TNG)
300,000,000 km
(3.28083989501 x 10^11 yds)
+67 Off chart Ringworld

As a Limitation

  • Size (allows ST and HP to be bought cheaper)

Also see Large-Area Injury and Trampling

Replaces Classic's Inconvenient Size

Giant Versions of Creatures

This is based on Behind the Curtain: How Strong Is a Giant? guidelines[3] The Weight multiplier cube root adjust ST and HP. "For weight multipliers greater than 1,000, divide by 1,000, multiply the resulting height multiple times 10, and add 6 to the resulting SM."

SM Increase (+1) (+2) (+3) (+4) (+5) (+6)
1.5x 2x 3x 5x 7x 10x
3.2x 10x 32x 100x 320x 1000x
cube root
~1.47x ~2.15x ~3.17x ~4.64x ~6.84x 10x

Using the Table

You want to create a giant rat using the brown rat (SM -4; ST/HP 2; 16" (~0.0278 hex); 0.31 – 1.1 lbs) as the foundation. You decide to make it man size (SM 0) ala Doctor Who's The Talons of Weng-Chiang. That is +4 to SM which is 5 x height/length, 100x weight, or 80" (~6.6'), 31 to 110 lbs,, ST/HP 10.

in Powers

Relative size is not immediately addressed, but as Kromm highlights in it is discussed on page 76 of GURPS Powers. He suggests a 4 cap as well as a house rule.

This is found under Shrinking called Small Size and Combat, which actually includes seven rules (it is the last):

Perception checks to defend

At SM -10 or smaller, when he attacks, his target must make a Vision roll. Modifiers include the attacker’s SM and the +10 to spot something in plain sight (see Vision, p. B358); e.g., SM -14 requires a Vision-4 roll.
Failure means the defender doesn’t see the attack coming and gets no defense, as if his attacker were invisible.

Hitting Chinks In Armor

If using Targeting Chinks in Armor, reduce the penalty to hit by SM, to a minimum of 0. Those with SM -10 or smaller can target gaps in armor at no penalty.[4]

Bypassing Natural DR

By making a successful grapple, a fighter with SM -13 or smaller can crawl inside unsealed armor, where he can ignore its DR. If he can reach a nostril, ear, or other orifice, a successful grapple lets him get inside his foe’s body, where he can also ignore natural DR.
At SM-19, they can get into the bloodstream via wound, and at SM-44, a creature is small enough to float through the pores on ordinary skin! All of these methods are primarily used to damage Vitals--or, when even smaller, delivering viruses or nanobots!
These sizes are most likely to be useful when playing a shrink ray, into-the-body, Magic School Bus-style adventure.

relative size

In battles between tiny combatants, the difference in SM is what matters.
For instance, if an SM -15 super is fighting an SM -10 foe, he applies these rules as if he had SM -5 and his enemy had SM 0.

over-riding basic

Although one might "these rules" as only referring to the NEW rules, it pretty much makes sense to apply this to the general bonus/penalty system on B19 too. IE/...

  • a SM+1 guy should be only +1 (not +2) to hit the SM+2 guy and the SM+2 guy should be -1 (not +1) to hit the SM+1 guy
  • a SM-1 guy should be only -1 (not -2) to hit a SM-2 guy while the SM-2 guy should be +1 (not -1) to hit the SM-1 guy

Using TDIS (The Difference in Size) rules helps avoid situations like giants who almost always hit each other and mice who almost never hit each other.

FAQ How do I apply Size Modifiers in melee combat? Is a large SM anything other but trouble?

Normally, the size modifier of your opponent is applied to your attack roll, whatever yours is. The problem is in some cases, it causes a big problem; for example, two ants (SM -10) have practically no chance of hitting each other unless they have Brawling-20+. To correct this, a new optional rule has been introduced:

When two opponents face each other in melee combat, apply the difference in size modifiers as a bonus to the smallest attacker and as a penalty to the biggest. However, the bonus to a small opponent is limited to +4; attacking a wall ten times as big as you is no easier than hitting a wall four times as big – your reach limits where you can hit.

Note: This is an addendum to the rule in the Basic Set, not an erratum.

Very optional rule:

even though being closer than 2 yards normally gives no bonus to ranged attacks, a GM may declare that being closer than 2 yards gives bonuses only to offset the size penalty. That way two characters will have the same chance to hit if their proportions (size/distance between) are the same. Warning: I couldn't find a Krommquote for this one, but it solves some problems.

And before you think that big SM is always bad in combat, consider the Features section below!


You can buy high ST (including Arm ST, Lifting ST and Striking ST) and HP at -10% discount per +1 SM, up to -80% (pp.B15-16).

When you try Intimidation skill against a smaller-sized victim, you get +1 per the difference in SM (p.B202).

Any weapon you're holding has longer Reach than usual[5] When you try to grapple a smaller opponent, you always get +1 per the difference in SM.[5]

When you try to pin a smaller opponent, you always get +3 per the difference in SM.[6]

If you are larger than your foe by 2+ SM (or you are larger than your foe by 1+ SM and he is lying prone), you can attempt trampling, which can be avoided only by Dodging.[7]

If your SM is 1 or more, Regular (friendly or hostile) spells cost (1+SM) times FP than usual to affect you.[8] This can be useful for spell defense, but makes helpful enchantments more difficult!

If your SM is 1 or more, poisons take (2 to the SM-th power) times longer to affect you.[9]

While larger size requires larger cover, you also gain the ability to shoot over larger obstacles. Also notice that crossing an SM-3 (relative to your SM) obstacle is easy.

You get to use larger weapons (this is most relevant in DF, but bigger guns could be an advantage too).

Technical Grappling

If using Technical Grappling, treat the victim’s limbs or neck as two SM smaller.[10]



In 2005:

The reason why small animals seem to evade well is because, usually, we're trying to grab them. SM applies as usual when grappling; if you try to grapple a mouse, you suffer its SM on your attack roll just as you would on any attack roll, and you grapple at -9 or so. Even if you succeed, the mouse may dodge. In plain English, the mouse has "evaded" you if you fail.

However, it hasn't evaded you in GURPS terms -- you just missed, or it dodged. The mouse would only have evaded you in game terms if it had tried to run over your body or between your legs and you had failed to swat it back.[11]

Kromm seems to ignore the high SM bonus to grappling low SM foes, presumably this offsets at least some of this penalty.

In 2013 at a breakdown

High SM is good because it . . .

  • allows you to hurdle larger objects as a free action (p. B352).
  • enables trampling (p. B404) and overruns (p. B432).
  • entitles you to buy cheaper ST (p. B15), Arm ST (p. B40), Lifting ST (p. B65), Striking ST (p. B88), and HP (p. B16).
  • gives you extra reach in melee combat (p. B402) and lets you fight at a height advantage (p. B403).
  • grants bonus DX and ST when grappling, whatever grappling rules you prefer.
  • makes you more costly to affect with harmful Regular spells (p. B239).
  • means you have no penalty to wield higher-SM melee weapons that enjoy improved damage, whichever weapon-scaling rules you prefer.
  • renders you more intimidating and less likely to be intimidated (p. B202), and allows you to shrug off the fear effect of being charged (p. B397).
  • slows the effects of poison (p. B438).

High SM is bad because it . . .

  • amplifies the effects of being crushed by high pressure (p. B435).
  • causes you to be hit by more fragments from explosions (p. B415).
  • leaves you more susceptible to high-RoF attacks when motionless (p. B409).
  • makes you easier to hit (p. B19), whatever version of the rules you prefer.
  • makes you more costly to affect with helpful Regular spells (p. B239).
  • means you must pay more to eat, if using the optional rules for that.
  • raises the cost and weight of your gear (p. B20).
  • renders you easier to see (p. B19).

Relative Size Modifier

A list by munin:

Higher SM

Pro Buy ST, HP, Arm ST, Lifting ST, Striking ST at reduced cost (SM +1 or more, p. B15, 16, 40, 66, 89)
Pro Immune to Constriction Attack (relative SM +1 or more, p. B43)
Pro Easier to intimidate, harder to be intimidated (p. B202)
Pro Easier to pin, harder to be pinned (p. B370)
Pro Can squeeze torso (relative SM +1 or more, p. B371)
Pro Increased reach (SM +1 or more, p. B402)
Pro Easier to hit when you grapple (p. B402)
Pro Can trample/overrun (relative SM +2 or more, p. B404, 432)
Pro Longer poison delay (p. B438)
Pro Eat less frequently (p. BIO63)
Pro Larger weapons do more damage (p. DF1:27)
Pro Unliving, Homogenous, and Diffuse targets add SM to resistance roll vs. Afflictions (p. P40)
Pro "Realistic" larger creatures eat proportionately less amounts of food† (p. S149)
Con Easier to be hit (p. B19)
Con Easier to be noticed (p. B19) by Vision (p. B358) or Vibration Sense (p. B96)
Con Vulnerable to Parasitic Possession (relative SM +1 or more, p. B76)
Con Harder to be camouflaged (SM +1 or more, p. B183)
Con More damage from extreme pressure (SM +2 or more, p. B435)
Con Eat more food† (pp. BIO62-63)
Con Require more space and life support (p. BIO63)
Con Larger equipment costs more and weighs more (p. BIO63, p. DF1:28, p. HT10, p. P50, p. UT16)
Situational Increases the size of a Jumper Tunnel (p. B64) or Permeation Tunnel (p. B75)
Situational Costlier to be affected with Regular spells (SM +1 or more, p. B239)
Situational More likely to block line of sight (p. B389)
Situational Attack counts as Large-Area Injury* (relative SM +7 or more, p. B400)

In addition, higher SM can be justification for some traits, but you'd have to actually pay for the traits themselves:

Pro "Realistic" larger creatures have higher ST (p. BIO64)
Pro "Realistic" larger creatures have longer lifespans (p. S159)
Pro "Realistic" larger creatures might more easily see low wavelengths (p. S162)
Con "Realistic" larger creatures might have a penalty to their sense of touch (p. S162)

Lower SM

Pro Harder to be hit (p. B19)
Pro Harder to be noticed (p. B19) by Vision (p. B358) or Vibration Sense (p. B96)
Pro Immune to Parasitic Possession (relative SM +0 or less, p. B76)
Pro Undetectable by Radar (SM -1 or less, p. B81)
Pro Simpler to grapple a prone, kneeling, or sitting opponent (relative SM -2 or less, p. B370)
Pro Eat less food† (pp. BIO62-63)
Pro Require less space and life support (p. BIO62)
Pro Smaller equipment costs less and weighs less (p. DF3:8, p. HT10, p. UT16)
Pro Attacks require Vision roll to notice (OPTIONAL, relative SM -10 or less, p. P76)
Pro Easier to target armor chinks (OPTIONAL, p. P76)
Pro Easier to get inside armor/body (OPTIONAL, relative SM -13/19 or less, p. P76)
Con Vulnerable to Constriction Attack (relative SM +0 or less, p. B43)
Con Harder to intimidate, easier to be intimidated (p. B202)
Con Harder to pin, easier to be pinned (p. B370)
Con Shorter poison delay (p. B438)
Con Eat more frequently (p. BIO63)
Con Smaller armor provides less DR, smaller weapons do less damage (p. DF3:8)
Con Impossible to target out-of-reach hit locations (OPTIONAL, p. P76)
Con "Realistic" smaller creatures eat proportionately greater amounts of food† (p. S149)
Situational Reduces the size of a Jumper Tunnel (p. B64) or Permeation Tunnel (p. B75)
Situational Less likely to block line of sight (p. B389)

In addition, lower SM can be justification for some traits, but you'd have to actually pay for the traits themselves:

Pro "Realistic" smaller creatures can carry heavier proportional loads (p. F51)
Pro "Realistic" smaller creatures might have a bonus to their sense of touch (p. S162)
Con "Realistic" smaller creatures have lower ST (p. BIO63)
Con "Realistic" smaller creatures have lower IQ (p. BIO63)
Con "Realistic" smaller creatures have shorter lifespans (p. S159)
Situational "Realistic" smaller creatures have lower weight (FEATURE, p. BIO63)


Bruno did an interesting breakdown where it seems like a disadvantage:

See Also


Ups and Downs:



Borderlands/Size Modifier OttGaming Wiki (IA)

Errata Notes

  • B19 discusses bonuses and penalties to hit those of higher or lower SM. This lists different amounts than the Size and Speed/Range Table on B550!
  1. between 11.4' and 11.5' is where the boundary actually is
  1. Basic Set, p.402
  2. Basic Set pg 550
  3. GURPS Fantasy (Behind the Curtain: How Strong Is a Giant?) pg 51
  4. Basic Set, p.400
  5. 5.0 5.1 Basic Set, p.402
  6. Basic Set, p.370
  7. Basic Set, p.404
  8. Basic Set, p.239
  9. Basic Set, p.438
  10. Technical Grappling, p.27
  11. Forum post here