Secret Lesson 4

D20 Tests: The Core Mechanic of the Game

You now understand the six abilities and the modifiers derived from them.

The next step is learning how those numbers are actually used during play.

At the center of the 5E rules system is one extremely important mechanic:

THE D20 TEST

Whenever the outcome of an action is uncertain and the rules call for a roll, the game often uses a twenty-sided die, or d20, to determine whether the attempt succeeds or fails.

The SRD groups three major kinds of rolls under the term D20 Test:

Ability Checks

Saving Throws

Attack Rolls

 

These three rolls appear constantly throughout the game.

If you understand the basic structure of a D20 Test, you already understand the mathematical foundation behind a large part of 5E.

WHAT IS A D20?

A d20 is a twenty-sided die.

Its faces are numbered from:

1 to 20

When you roll it, each face represents a possible result.

In a D20 Test:

higher is generally better.

The die gives you the starting number.

Then the rules may tell you to add or subtract modifiers.

Finally, you compare the total with a target number.

THE BASIC D20 TEST FORMULA

The core idea can be summarized like this:

d20 roll + relevant modifiers = total

Then:

Compare the total with the target number.

If your total equals or exceeds the target number:

the D20 Test succeeds.

If your total is lower:

the D20 Test fails.

That simple procedure powers a huge number of situations in the game.

EXAMPLE

Suppose the target number is:

15

You roll:

12

Your relevant modifiers add:

+4

So:

12 + 4 = 16

Because:

16 ≥ 15

the test succeeds.

ANOTHER EXAMPLE

Target number:

15

You roll:

9

Modifiers:

+3

Total:

12

Because:

12 < 15

the test fails.

The basic question is always:

DID THE FINAL TOTAL REACH THE TARGET NUMBER?

WHEN IS A D20 TEST USED?

The SRD introduces D20 Tests as the game's method for determining success or failure when an outcome is uncertain.

This connects directly to the rhythm of play we studied earlier.

The player describes what the character does.

The GM determines how the situation should be resolved.

If uncertainty requires a D20 Test, the player rolls.

The roll helps determine what happens next.

NOT EVERY ACTION IS A D20 TEST

This is important.

The game does not require a d20 roll every time a character does something.

Imagine you say:

“I walk across the room.”

If nothing makes that difficult or uncertain, there may be no reason to roll.

But imagine you say:

“I try to leap across the collapsing bridge before it falls.”

Now the result might be uncertain.

The rules or the GM may call for a D20 Test.

The dice are there to resolve uncertainty, not to interrupt every ordinary action.

THE THREE TYPES OF D20 TEST

Every D20 Test belongs to one of three categories.

Understanding the distinction is essential.

1. ABILITY CHECKS

An Ability Check represents a creature using its abilities, talent, and training to overcome a challenge.

Examples given by the SRD include attempting things such as:

  • forcing open a stuck door

  • picking a lock

  • entertaining a crowd

  • deciphering a cipher

Ability Checks are commonly used for activities other than attacks when failure is meaningful and the result is uncertain.

For example:

Can you climb the difficult surface?

Can you find the hidden clue?

Can you persuade the suspicious guard?

Can you remember the significance of an ancient symbol?

These can involve Ability Checks.

We will study them properly in:

LESSON 5 — ABILITY CHECKS, SKILLS, AND DIFFICULTY CLASSES

2. SAVING THROWS

A Saving Throw, often shortened to save, represents an attempt to resist or evade a threat.

The SRD gives examples such as:

  • avoiding a fiery explosion

  • enduring poisonous gas

  • resisting a spell that attacks the mind

Unlike many Ability Checks, you normally don't decide to make a Saving Throw.

Something dangerous happens to you, and the effect requires you to make one.

For example:

A spell explodes around you.

The rules might call for a Dexterity Saving Throw.

Poison enters your system.

You might need a Constitution Saving Throw.

Magic attacks your mind.

You might need a Wisdom Saving Throw.

We will study Saving Throws in:

LESSON 6 — SAVING THROWS: RESISTING THREATS, HAZARDS, AND EFFECTS

3. ATTACK ROLLS

An Attack Roll determines whether an attack hits its target.

You roll the d20, apply the appropriate modifiers, and compare the final result with the target's Armor Class, or AC.

If the total equals or exceeds the target's AC:

the attack hits.

Otherwise:

the attack misses.

Attack Rolls normally occur during combat, although the SRD notes that they can also appear in other situations, such as an archery competition.

We will study Attack Rolls and Armor Class in detail in:

LESSON 7 — ATTACK ROLLS, ARMOR CLASS, NATURAL 20s, AND NATURAL 1s

THE SAME BASIC ENGINE

Notice something important.

An Ability Check, Saving Throw, and Attack Roll represent very different things.

But mechanically, they all use the same core system:

ROLL A D20

ADD THE APPROPRIATE MODIFIERS

COMPARE THE TOTAL WITH A TARGET NUMBER

That is why the SRD groups all three together under one name:

D20 TESTS

Once you understand this engine, many different rules become easier to learn.

STEP 1 — ROLL THE D20

The first part is simple:

Roll one d20.

You generally want the result to be as high as possible.

Imagine you roll:

14

That is your starting number.

But the die result is usually not the complete result.

Next come the modifiers.

STEP 2 — ADD THE RELEVANT ABILITY MODIFIER

Every D20 Test uses a relevant Ability Modifier.

Remember the six abilities:

Strength

Dexterity

Constitution

Intelligence

Wisdom

Charisma

The rules determine which one applies to the particular D20 Test.

Suppose you roll:

14

and the relevant ability modifier is:

+3

Then:

14 + 3 = 17

Your current total is:

17

WHY THE RELEVANT ABILITY CHANGES

Different challenges test different capabilities.

For example:

Trying to force something open might use Strength.

Moving with agility might use Dexterity.

Enduring physical strain might use Constitution.

Reasoning through information might use Intelligence.

Noticing something might use Wisdom.

Influencing someone might use Charisma.

The details of these choices will become much clearer in Lesson 5.

For now, remember:

every D20 Test has a relevant ability.

STEP 3 — ADD YOUR PROFICIENCY BONUS IF RELEVANT

Sometimes you also add your Proficiency Bonus.

The SRD describes Proficiency Bonus as a number a creature adds when a D20 Test uses something in which that creature has proficiency.

This does not mean that Proficiency Bonus is added to every D20 Test.

It is added only when it is relevant.

Suppose:

d20 roll = 12

relevant Ability Modifier = +3

Proficiency Bonus = +2

and your proficiency applies.

Then:

12 + 3 + 2 = 17

Your final total is:

17

WHAT DOES PROFICIENCY REPRESENT?

At a basic level, proficiency represents training or particular capability with something recognized by the rules.

This can include areas such as:

  • skills

  • saving throws

  • weapons

  • tools

Different rules determine when proficiency applies.

We will study the full system later in:

LESSON 9 — PROFICIENCY: PROFICIENCY BONUS, SKILLS, SAVING THROWS, WEAPONS, TOOLS, AND EXPERTISE

For now, remember:

Proficiency Bonus is not automatic.

You add it only when the D20 Test involves something for which the rules say your proficiency applies.

STEP 4 — APPLY OTHER BONUSES OR PENALTIES

The SRD also allows other rules to modify a D20 Test.

A:

  • class feature

  • spell

  • other rule

might provide an additional bonus or impose a penalty.

Suppose you have:

d20 = 11

Ability Modifier = +3

Proficiency Bonus = +2

special bonus = +1

Then:

11 + 3 + 2 + 1 = 17

If another effect imposed a penalty of −2:

17 − 2 = 15

The final number after all applicable modifiers is the number you compare with the target.

THE COMPLETE STRUCTURE

A useful general formula is:

d20

+ Relevant Ability Modifier

+ Proficiency Bonus, if relevant

+ Other bonuses

− Applicable penalties

= D20 Test Total

Then:

Compare that total with the target number.

STEP 5 — COMPARE WITH THE TARGET NUMBER

Once the modifiers have been applied, the final number is compared with a target.

If:

Total ≥ Target

the D20 Test succeeds.

If:

Total < Target

the D20 Test fails.

Notice that you do not need to beat the target number.

Matching it is enough.

EXACTLY EQUAL IS A SUCCESS

Suppose the target is:

DC 15

Your total is:

15

You succeed.

You do not need:

16

The SRD rule is:

equal to or greater than.

This is an important beginner rule.

TARGET NUMBERS: DC AND AC

D20 Tests use two major kinds of target number.

DIFFICULTY CLASS — DC

For:

Ability Checks

and

Saving Throws

the target number is called a:

DIFFICULTY CLASS

usually written:

DC

 

ARMOR CLASS — AC

For:

Attack Rolls

the target number is called:

ARMOR CLASS

usually written:

AC

 

So:

Ability Check → DC

Saving Throw → DC

Attack Roll → AC

This distinction will appear throughout the entire course.

EXAMPLE — ABILITY CHECK

Imagine a challenge has:

DC 15

Your roll:

11

Relevant Ability Modifier:

+3

Applicable Proficiency Bonus:

+2

Total:

11 + 3 + 2 = 16

Because:

16 ≥ 15

you succeed.

EXAMPLE — SAVING THROW

A dangerous effect requires:

DC 14

You roll:

9

Relevant Ability Modifier:

+2

Applicable Proficiency Bonus:

+2

Total:

13

Because:

13 < 14

the Saving Throw fails.

What happens after the failure depends on the effect that caused the save.

EXAMPLE — ATTACK ROLL

Your target has:

AC 16

You roll:

13

Relevant Ability Modifier:

+3

Proficiency Bonus:

+2

Total:

18

Because:

18 ≥ 16

the attack hits.

What damage the attack deals is a separate step that we will study later.

THE GM AND TARGET NUMBERS

The SRD explains that the Game Master determines target numbers and tells players whether their rolls succeed.

In many situations, a rule already supplies the relevant number.

In others, particularly Ability Checks, the GM may determine the appropriate Difficulty Class.

We will study how Difficulty Classes work in the next lesson.

YOU DON'T ALWAYS NEED TO KNOW THE TARGET NUMBER

A player can often make a D20 Test without necessarily knowing the exact DC.

For example, the GM might simply say:

“Make a Wisdom check.”

You roll.

Add the appropriate modifiers.

Tell the GM your total.

The GM compares that result with the target number and describes what happens.

ADVANTAGE AND DISADVANTAGE

Sometimes circumstances modify the way the d20 itself is rolled.

The SRD calls these mechanics:

Advantage

and

Disadvantage

With Advantage or Disadvantage, two d20s are rolled, and only one result is used:

Advantage → use the higher result

Disadvantage → use the lower result

We are deliberately not going deeper into those rules yet.

They receive their own full lesson:

LESSON 8 — ADVANTAGE, DISADVANTAGE, REROLLS, AND HEROIC INSPIRATION

For now, simply recognize that Advantage and Disadvantage can modify the first stage of a D20 Test.

A COMPLETE D20 TEST EXAMPLE

Imagine your adventurer is attempting something uncertain.

The GM calls for a D20 Test.

STEP 1 — ROLL

You roll:

13

STEP 2 — ABILITY MODIFIER

The relevant ability gives:

+2

Current total:

15

STEP 3 — PROFICIENCY

Your proficiency applies:

+3

Current total:

18

STEP 4 — OTHER MODIFIERS

A magical effect gives:

+1

Final total:

19

STEP 5 — COMPARE

Target:

DC 17

Final result:

19

Therefore:

SUCCESS

The story continues based on that successful result.

ANOTHER COMPLETE EXAMPLE

Suppose:

d20 roll = 7

Ability Modifier = +4

Proficiency does not apply

Penalty = −1

Your total is:

7 + 4 − 1 = 10

Target:

DC 12

Because:

10 < 12

the test fails.

This shows why the d20 result is only one part of the final calculation.

THE DIE AND THE CHARACTER BOTH MATTER

The D20 Test combines two elements.

THE DIE

Represents uncertainty.

THE MODIFIERS

Represent relevant game statistics, training, features, and circumstances.

A character with a strong modifier can still roll poorly.

A character with a weak modifier can still roll well.

That uncertainty is one of the fundamental elements of the system.

WHY A +1 MATTERS

Because D20 Tests compare a total against a target number, even a small modifier can change the outcome.

Suppose the target is:

15

You roll:

12

With:

+2

your result is:

14 — failure

But with:

+3

your result is:

15 — success

That single point changed the result.

THE D20 TEST IS A UNIVERSAL LANGUAGE

Once you understand:

roll → modify → compare

you can understand the basic structure behind:

  • climbing challenges

  • social challenges

  • resisting spells

  • avoiding hazards

  • attacking enemies

  • finding hidden things

  • many class abilities

  • many monster abilities

The circumstances change.

The relevant Ability Modifier changes.

Proficiency may or may not apply.

The target number changes.

But the central mechanism remains recognizable.

THREE QUESTIONS TO ASK WHEN MAKING A D20 TEST

Whenever you are asked to make one, ask yourself:

1. WHAT TYPE OF D20 TEST IS THIS?

Ability Check?

Saving Throw?

Attack Roll?

2. WHAT MODIFIERS APPLY?

Which Ability Modifier?

Does my Proficiency Bonus apply?

Is there another bonus or penalty?

3. WHAT TARGET MUST I REACH?

DC?

AC?

Once those questions are answered, the roll is usually straightforward.

DON'T MIX UP THE THREE TESTS

Although all three use the same basic d20 system, they are not interchangeable.

An Ability Check is not a Saving Throw.

A Saving Throw is not an Attack Roll.

An Attack Roll is not an Ability Check.

A rule that affects one type does not automatically affect another unless the wording says it does.

If a feature says:

“You gain a bonus to Ability Checks…”

that does not automatically mean you gain the same bonus to Attack Rolls.

If something affects:

all D20 Tests

then it can affect all three types.

This terminology matters.

D20 TESTS AND THE SIX ABILITIES

Lesson 3 taught you:

Ability Score → Ability Modifier

Now we can extend the chain:

Ability Score

↓

Ability Modifier

↓

D20 Test

↓

Compare with DC or AC

↓

Success or Failure

This is one of the central mechanical structures of the entire game.

LESSON 4 — KEY TAKEAWAYS

A D20 TEST RESOLVES UNCERTAINTY

The game uses D20 Tests to determine success or failure when an uncertain situation calls for one.

THERE ARE THREE TYPES

Ability Checks

Saving Throws

Attack Rolls

THE CORE PROCEDURE IS:

1. Roll the d20

2. Add the relevant Ability Modifier

3. Add Proficiency Bonus if it applies

4. Apply other bonuses or penalties

5. Compare the result with the target number

HIGHER IS BETTER

You normally want a high d20 result.

MATCHING THE TARGET IS ENOUGH

If:

Total = Target

you succeed.

You don't need to exceed it.

ABILITY CHECKS AND SAVING THROWS USE DC

DC = Difficulty Class

ATTACK ROLLS USE AC

AC = Armor Class

PROFICIENCY IS CONDITIONAL

You add Proficiency Bonus only when the relevant rules say your proficiency applies.

OTHER RULES CAN MODIFY THE ROLL

Features, spells, and other effects can provide bonuses or penalties.

ADVANTAGE AND DISADVANTAGE MODIFY THE D20 ROLL

They involve rolling two d20s and using one of them.

Their complete rules will come later.

NEXT LESSON

LESSON 5 — ABILITY CHECKS, SKILLS, AND DIFFICULTY CLASSES

You now understand the universal structure of a D20 Test.

Next, we will examine the first of its three forms in detail:

THE ABILITY CHECK

We will learn:

  • when the GM calls for an Ability Check

  • which ability should be used

  • how Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma checks differ

  • how skill proficiencies interact with checks

  • how tool proficiencies can apply

  • when Proficiency Bonus is added

  • what a Difficulty Class represents

  • the standard DC scale from very easy to nearly impossible

  • how the GM determines whether a check is appropriate

This is where the D20 Test begins to become a practical tool for exploration, social interaction, problem-solving, and countless other actions.

SRD 5.2.1 ATTRIBUTION

This work includes material from the System Reference Document 5.2.1 (“SRD 5.2.1”) by Wizards of the Coast LLC, available at https://www.dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License, available at https://creativecommons.org/licenses/by/4.0/legalcode.