Checks

Start by rolling your d20. You'll then identify all the relevant modifiers, bonuses, and penalties that apply. A modifier can be either positive or negative, but a bonus is always positive, and a penalty is always negative. The sum of all the modifiers, bonuses, and penalties you apply to the d20 roll is called your total modifier for that statistic.

This step is simple. Add up all the various modifiers, bonuses, and penalties you identified in Step 1—this is your total modifier. Next add that to the number that came up on your d20 roll. This total is your check result.

Whenever you attempt a check, you compare your result against the Difficulty Class (DC) of the check. Your check succeeds if it's equal to or greater than the DC. If you roll anything less than the DC, you fail.
Sometimes you'll know the DC and make the comparison yourself. Other times, you might not know the DC right away. Unlocking a looted comm unit would require a Computers check, but it doesn't have a specified DC—so how will you know if you succeed or fail? You call out your result to the GM and they'll let you know if it's a success, failure, or otherwise. While you might learn the exact DC through trial and error, DCs sometimes change, so asking the GM whether a check is successful is best.

Many times, it's important to determine not only if you succeed or fail, but also how spectacularly you succeed or fail. Exceptional results—either good or bad—can cause you to critically succeed or critically fail at a check.
You critically succeed when the check's result meets or exceeds the DC by 10 or more. If the check is an attack roll, this is also known as a critical hit. You can also critically fail a check. The rules for critical failure—sometimes called a fumble—are the same as those for a critical success, but in the other direction: if you fail a check by 10 or more, that's a critical failure.
Some actions and abilities have stronger effects on a critical success or failure. For example, a Strike deals double damage on a critical hit. If an effect doesn't list a critical success effect, the critical success effect is the same as the success effect, and the same goes for critical failures.

When you use a Strike action or make a spell attack, you attempt a check called an attack roll. Attack rolls take a variety of forms and are often highly variable based on the weapon you are using for the attack, but there are three main types: melee attack rolls, ranged attack rolls, and spell attack rolls. Spell attack rolls work a little bit differently, so they are explained separately on the next page.
Melee attack rolls use Strength as their attribute modifier by default. If you're using a weapon or attack with the finesse trait, then you can use your Dexterity modifier instead.
Melee attack roll result = d20 roll + Strength modifier (or optionally Dexterity modifier for a finesse weapon) + proficiency bonus + other bonuses + penalties
Ranged attack rolls use Dexterity as their attribute modifier.
Ranged attack roll result = d20 roll + Dexterity modifier + proficiency bonus + other bonuses + penalties
When attacking with a weapon, whether melee or ranged, you add your proficiency bonus for the weapon you're using. Your class determines your proficiency rank for various weapons. Sometimes, you'll have different proficiency ranks for different weapons or groups of weapons.
The bonuses you might apply to attack rolls can come from a variety of sources. Circumstance bonuses can come from the aid of an ally or a beneficial situation. Status bonuses are typically granted by spells and other magical aids. The item bonus to attack rolls comes from tracking weapons.
Penalties to attack rolls come from situations and effects as well. Circumstance penalties come from risky tactics or detrimental circumstances, status penalties come from spells and magic working against you, and item penalties occur when you use a shoddy item. When making attack rolls, two main types of untyped penalties are likely to apply. The first is the multiple attack penalty, and the second is the range penalty. The first applies any time you make more than one attack action during the course of your turn, and the other applies only with ranged or thrown weapons. Both are described below.

If you cast spells, you might need to make a spell attack roll. These rolls are usually made when you cast a spell that targets a creature's AC.
The attribute modifier for a spell attack roll depends on how you gained the spell. If your class grants you spellcasting, use your key attribute modifier (such as Wisdom for a mystic). Innate spells use your Charisma modifier unless the ability that granted them states otherwise. Focus spells and other sources of spells specify which attribute modifier you use for spell attack modifiers in the ability that granted them. If you have spells from multiple sources or traditions, you might use different attribute modifiers for spell attack modifiers for these different sources of spells. For example, a lashunta healing mystic with the Psychic Talent ancestry feat would use their Charisma modifier when casting a cantrip from that feat since it's an occult innate spell, but they would use their Wisdom modifier when casting infusion and other spells using their mystic divine spellcasting.
If you have the ability to cast spells, you'll have a proficiency rank for your spell attack modifier, so you'll always add a proficiency bonus. Spell attack rolls can benefit from circumstance bonuses and status bonuses, though item bonuses to spell attack rolls are rare. Penalties affect spell attack rolls just like any other attack roll—including your multiple attack penalty.
Calculate a spell attack roll with the following formula.
Spell attack roll result = d20 roll + spellcasting attribute modifier + proficiency bonus + other bonuses + penalties

Many times, instead of requiring you to make a spell attack roll, the spells you cast require those within the area or targeted by the spell to attempt a saving throw against your spell DC to determine how the spell affects them. Calculate it using the following formula.
Spell DC = 10 + spellcasting attribute modifier + proficiency bonus + other bonuses + penalties

Multiple statistics help you defend against attacks, spells, hazards, and other dangers of adventuring: Armor Class and three saving throws.

Perception measures your ability to be aware of your environment. Every creature has Perception, which works with and is limited by a creature's senses. (Details on senses and detecting things begin here.) Whenever you need to attempt a check based on your awareness, you'll attempt a Perception check. Your Perception uses your Wisdom modifier, so you'll use the following formula when attempting a Perception check.
Perception check result = d20 roll + Wisdom modifier + proficiency bonus + other bonuses + penalties
Nearly all creatures are at least trained in Perception, so you will almost always add a proficiency bonus to your Perception modifier. You might add a circumstance bonus for advantageous situations or environments and typically get status bonuses from spells or other magical effects. Items can also grant you a bonus to Perception, typically in a certain situation. For instance, the retinal reflectors augmentation grants a +1 item bonus to Perception checks to Seek.
Circumstance penalties to Perception occur when an environment or situation (such as fog) hampers your senses, while status penalties typically come from conditions, spells, and magic effects that foil the senses. You'll rarely encounter item penalties or untyped penalties for Perception.
Many abilities are compared to your Perception DC to determine whether they succeed. As with any DC based on a modifier, your Perception DC is 10 + your total Perception modifier.

Starfinder has a variety of skills for many different situations, from Athletics to Computers to Piloting. Each grants you a set of related actions that rely on you rolling a skill check. Each skill has a key attribute modifier, based on the scope of the skill in question. For instance, Athletics deals with feats of physical prowess, like swimming and jumping, so its key attribute modifier is Strength. Computers deals with the ability to work with computer systems and other technological equipment, so its key attribute modifier is Intelligence. The key attribute modifier for each skill is listed in Chapter 4: Skills. No matter which skill you're using, you calculate a check for it using the following formula.
Skill check result = d20 roll + the skill's key attribute modifier + proficiency bonus + other bonuses + penalties
You're unlikely to be trained in every skill. As normal, when using a skill in which you're untrained, your proficiency bonus is +0; otherwise, it equals your level plus 2 for trained, or higher once you become expert or better. The proficiency rank is specific to the skill you're using. Aid from another character or some other beneficial situation might grant you a circumstance bonus. A status bonus might come from a helpful spell or magical effect. Sometimes tools related to the skill grant you an item bonus to your skill checks. Conversely, unfavorable situations might give you a circumstance penalty to your skill check, while harmful spells, magic, or conditions might also impose a status penalty. Using shoddy or makeshift tools might cause you to take an item penalty. Sometimes a skill action can be an attack, and in these cases, the skill check might take a multiple attack penalty.
When an ability calls for you to use the DC for a specific skill, you can calculate it by adding 10 + your total modifier for that skill.