How do I read 2d10 as a percentage?

Question

I own 2 d10 dice with differently numbered sides (0 through 9, and 00 through 90), although the same applies for two identical 'regular' d10 dice.

How do I read the percentage value if I use both dice?

It's pretty straight-forward if I roll between 11 and 89, but outside this range it becomes slightly confusing:

Roll 1	Roll 2	Result
50	6	56%
80	9	89%
00	0	0% or 10% or 100%?
90	0	90% or 100%?
10	0	10% or 20%?

Note: if I roll with identical d10, I roll sequential, not simultaneous to avoid discussion if the 8 or 9 is the first digit.

I have a hunch that 0% isn't really a thing in most systems that use d100, so that eliminates 00, 0 → 0%.

So is it then true that on the first d10, the 00 counts as zero, but on the second d10, the 0 counts as ten?

Answer 1

You read one die as the 10s place and the other die as the 1s place. Traditionally, (00, 0) means 100 instead of 0. Your set is marked to make forgetting which die is which, intentionally or accidentally, impossible.

10s	1s	Reads as
00	1	1
00	2	2
10	0	10
30	1	31
50	6	56
80	9	89
90	0	90
90	1	91

If you have just two dice numbered 0-9 you can do the same thing by just designating one color as 10s and the other as 1s.

Apparently, there are some new d100 sets that are intended to be added together, so that 90, 10 = 100, but those are not the dice you have described.

Answer 2

Add face values:

X	00	10	20	30	40	50	60	70	80	90
0	00	10	20	30	40	50	60	70	80	90
1	01	11	21	31	41	51	61	71	81	91
2	02	12	22	32	42	52	62	72	82	92
3	03	13	23	33	43	53	63	73	83	93
4	04	14	24	34	44	54	64	74	84	94
5	05	15	25	35	45	55	65	75	85	95
6	06	16	26	36	46	56	66	76	86	96
7	07	17	27	37	47	57	67	77	87	97
8	08	18	28	38	48	58	68	78	88	98
9	09	19	29	39	49	59	69	79	89	99

If you need a range of 1%-100% instead of 0%-99%, interpret 0% as 100%

Answer 3

Starting from scratch: Percentile dice (d%) is meant to get you a number between 1-100. The "classic" way of getting this number is with two d10s, one designated as your "tens digit", the other as your "ones digit". (Some sets will give you a tens-digit dice that's actually labelled that way: instead of 1,2,3,4,5,6,7,8,9,0 it'll have 10,20,30,40,50,60,70,80,90,00. It's purely a help, doesn't change the mechanics at all.)

Before rolling, choose one of the two dice to be your "tens" digit. Roll them, then arrange them in order (just like you were writing it down - tens to the left of ones). Read the number, that's your answer. If you get a 0 in the tens spot, it means you rolled under 10.

The only weirdness is if you get 00. (Zeroes on both dice). That's 100. (Just imagine the little floating hundreds digit "1" there). If you're using a game that goes 0-99 rather than 1-100, then that's a straight up 0.

The important point is that there's no adding involved - you never add one die to the other, because each die is rolling a separate digit.

Answer 4

There is no single right way. The important point is that everybody knows how you do it before you roll. It might also be nicer, if everybody in the group rolls the same way, but it's not something to get into a fight over.

I prefer just adding the tens and ones, and 00+0 is 100. Arguably, it's also nicer when all zeros is the special number 100, instead of being obscure 10.

Some others in my group interpret 0 as 10, so 00+0 is 10, and 90+0 is 100. This might be the most logical choice if the dice actually were 1..10 and 00..90.

The important thing is correct probabilities, of course. As long as any percentage 1..100 can occur with equal probability, the most intuitive way is a matter of how one thinks. With 2 ten-sided dice, there's exactly 100 (10x10) permutations, so as long as every percentage 1..100 can come out of the throw, then each percentage has exactly one permutation, and all is fine.

Answer 5

So is it then true that on the first d10, the 0 counts as zero, but on the second d10, the 0 counts as ten?

That is a mathematically coherent way to do it, but most people just read it as a two-digit number, then translate 00 to 100. This also adds a bit of dramatic tension when rolling sequentially - if you roll a 0 first, then you're probably going to get a low score, but also have a chance of getting the best one.

If I had a die marked 1-10 (they do exist, but are very uncommon), I would probably add them together. Your system reminds me somewhat of the bijective system.

Answer 6

Before you roll, designate which of your two dice will represent the tens digit, and which die will represent the ones digit. This is easiest if the dice are visually distinct: two different colors, or one solid-color and one metallic, or different sizes or shapes, or whatever. But if you have some other way to keep track of which die is which, that will work too. For example, you could roll 1d10 twice, first for the tens digit and then for the ones digit.

Some companies also make a special "d%": a d10 specifically meant to represent tens digits. Instead of being numbered with the traditional 0-9, a d% is numbered using multiples of 10: 00, 10, 20, 30, 40, 50, 60, 70, 80, and 90. You can also use this as a standard d10 just by using only the tens digit on each face.

In any event, once you've decided which die is which digit, roll them both and combine them to get the result. If you're rolling 2d10, then multiply the tens die's result by 10 and then add them together. If you're rolling a d10 and a d%, then you can just add the numbers, because the die has already done the multiplying by 10 for you.

Lastly, remember that a zero result means 100, not 0. Even though you can roll a 0 on 1d10, you can't roll zero on 1d100.

Answer 7

It depends on the conventions you use for reading your dice.

You have a percentile dice (or a d10 you nominate as your percentile dice) and a d10 dice.

There are broadly three ways to read these dice, each with their own conventions. So long as you are consistent with your convention then any of these methods are functionally equivalent.

Method 1 - read the d10 as 0-9 and the percentile dice as d10*10 (ie 00, 10, 20, ... )

The natural sum of the two dice then gives you values from 0 - 99. This method won't be suitable for games like D&D (which want a range of 1-100)

Percentile Dice	Result		Regular Dice	Result
00	0		0	0
10	10		1	1
20	20		2	2
30	30		3	3
40	40		4	4
50	50		5	5
60	60		6	6
70	70		7	7
80	80		8	8
90	90		9	9

Example Calculations:

Percentile Dice	d10 dice	result
00	0	0 + 0 = 0
00	5	0 + 5 = 5
20	0	20 + 0 = 20
90	1	90 + 1 = 91
90	0	90 + 0 = 90

Method 2 - read the d10 as 0-9 and the percentile dice as d10*10 (ie 00, 10, 20, ... ), with a value of 00, 0 being 100

With this method we've introduced an exception for a roll of 00, 0. In particular, we've removed the lowest value from the previous set of sums.

Aside from the exceptional case we sum values on the dice as in method 1.

So now we get a range of 1-100 on the dice, with the lowest possible roll being 00, 1 giving a value of 1.

Percentile Dice	Result		Regular Dice	Result
00	0		0	0
10	10		1	1
20	20		2	2
30	30		3	3
40	40		4	4
50	50		5	5
60	60		6	6
70	70		7	7
80	80		8	8
90	90		9	9

Exceptions:

Percentile Dice	Regular Dice	Result
00	0	100

Example Calculations:

Percentile Dice	d10 dice	result
00	0	100
00	5	0 + 5 = 5
20	0	20 + 0 = 20
90	1	90 + 1 = 91
90	0	90 + 0 = 90

Method 3 - read the d10 as 1-10 (assigning the 10 to 0) and the percentile dice the same way as method 1

In this method we do a straight sum of the two dice to get a read.

This gives us a range of possible values of 1 - 100, with the lowest value being 1 (on a roll of 00,1)

Percentile Dice	Result		Regular Dice	Result
00	0		1	1
10	10		2	2
20	20		3	3
30	30		4	4
40	40		5	5
50	50		6	6
60	60		7	7
70	70		8	8
80	80		9	9
90	90		0	10

Example Calculations:

Percentile Dice	d10 dice	result
00	0	0 + 10 = 10
00	5	0 + 5 = 5
20	0	20 + 10 = 30
90	1	90 + 1 = 91
90	0	90 + 10 = 100

Answer 8

I have always rolled for percent in a mathematically consistent way. Your double digit die, whether it has double digits or not, goes from 0-90. The single digit die goes from 1-10, just as when rolling for damage. Using this system a 100 is achieved by rolling 90 and 0, 10 is added to 90 making 100. I prefer this since it truly has 100 parts and the value of each die is consistent.

Answer 9

The 00 means 00. So 00/1 is 1%, 00/2 is 2% and so on. The 0 counts as nothing. If you get both, and 0% is unacceptable, then that's your 100%.

However, if 0% is acceptable but 100% is not, then it's 0%. If none are acceptable, reroll. If both are acceptable, flip a coin. Heads is 100%, tails is 0%.

Answer 10

To me it, there is an easy to remember system to use depending on what dice you have available.

I think the problem stems from the fact that ten sided dice come either as 0-9 or 1-10 or 00-90.

From this you can potentially come up with six different combinations of these three different ten sided dice. But most people use one of three of these combinations to generate a number between 1 and 100:

1. 0-9 and 00-90

The result of the 00-90 dice represents the tens place and the 0-9 represents the ones place. The only exception to this rule is if you roll 00 and 0, which represents 100.

00 and 9 is 9

50 and 0 is 50

00 and 0 is 100

2. 1-10 and 00-90

Simply add the two dice results together.

00 and 1 is 1

10 and 10 is 20

90 and 10 is 100

3. 0-9 and 0-9

This one is covered in the PHB. Roll one dice, which represents tens place, roll the other, that represents the ones place. Double 0 is 100.

0 and 8 is 8

5 and 3 is 53

0 and 0 is 100

The other dice combinations get a bit more confusing. I would stick to one of these three.