Encounter building is the part of DMing that most people either over-think or skip entirely. The Dungeon Master's Guide dedicates several pages to XP thresholds, encounter multipliers, and daily budgets, but the core idea is simple: figure out how much danger your party can handle, then pick monsters that fit within that limit.
The XP threshold system explained
Every character level has four XP thresholds: easy, medium, hard, and deadly. A level 3 character, for example, has thresholds of 75, 150, 225, and 400 XP respectively. To get the party threshold, multiply the per-character value by the number of players. A party of four level 3 characters has a medium threshold of 600 XP. If the adjusted XP of your encounter sits between 600 and 900, it qualifies as a medium encounter.
Easy encounters burn minimal resources. They serve as warm-ups, ambience, or teaching moments for new players. Medium encounters have some teeth but rarely threaten a character's life. Hard encounters can go wrong quickly if dice and tactics misalign. Deadly encounters carry a genuine risk of one or more player characters dropping to zero hit points, and a catastrophic outcome is plausible if luck turns.
The encounter multiplier
A single monster worth 700 XP is not the same threat as seven monsters worth 100 XP each. The seven-monster group has seven turns per round, meaning it can focus damage, block movement, and pressure multiple PCs simultaneously. The DMG accounts for this by applying a multiplier to the total XP when comparing against thresholds. One monster has no multiplier. Two monsters multiply by 1.5. Three to six multiply by 2. Seven to ten multiply by 2.5. Eleven to fourteen multiply by 3. Fifteen or more multiply by 4.
This adjusted XP value is used for budgeting only. You still award the raw XP total if you use experience-based levelling. The distinction matters: a group of six goblins (300 raw XP) counts as 600 adjusted XP for difficulty purposes but only awards 300 XP to the party.
Challenge rating and what it actually means
Challenge rating is a rough indicator of how tough a monster is relative to a party. A CR 1 creature is meant to be a moderate threat to a party of four level 1 characters. In practice, CR is an approximation. Some creatures punch above their weight because of resistances, flight, or devastating special abilities. Others underperform because their stat blocks are straightforward. Treat CR as a starting line, not a finish line.
The XP value assigned to each CR is what actually matters for encounter building. A CR 5 creature is worth 1,800 XP. A CR 10 creature is worth 5,900 XP. The relationship between CR and XP is not linear, which is why eyeballing encounter balance without doing the arithmetic often leads to lopsided fights.
Practical encounter design tips
Mixing creature types makes encounters more interesting. A single ogre guarding a room is a speed bump. An ogre supported by four goblins who have half cover behind furniture is a tactical puzzle. The goblins force the party to split attention, the ogre punishes anyone who ignores it, and the furniture creates terrain decisions. All of that comes from combining creatures, not from increasing raw XP.
Terrain also shifts difficulty in ways the XP system cannot capture. Fighting a hill giant on an open plain is straightforward. Fighting the same hill giant on a narrow rope bridge above a chasm changes the encounter entirely without touching the XP math. Use this tool to get the numbers right, then layer in terrain, objectives, and time pressure to make the encounter memorable.
Common mistakes
The most frequent error is forgetting the encounter multiplier. Four CR 1 creatures (800 raw XP) seem fine for a level 3 party, but their adjusted XP is 1,600, which lands firmly in deadly territory. The second most common error is pitting a solo monster against a full party. A single creature, no matter how impressive its stat block, gets buried by action economy. Five PCs get five turns per round. One dragon gets one turn and maybe a legendary action. The math favours the many.
Another trap is running too many deadly encounters in a single adventuring day. The DMG's daily XP budget exists for a reason: it measures how much punishment a party can absorb before they need a long rest. Exceeding it means characters run out of spell slots, hit dice, and abilities, and what should be a tense final encounter becomes a death spiral.
Using this tool with virtual tabletops
Once you have your encounter, you need a place to run it. Roll20 and Foundry VTT both accept custom map images. MakeMythic battlemaps are designed for both platforms: drop the image in, set the grid, and populate with tokens. If you are using our grid overlay tool, export the gridded map at 70px per square for Roll20 or 100px per square for Foundry, then place your monsters according to the encounter you built here.
For in-person games, print the map at 1 inch per square (300 DPI) and use miniatures or cardboard standees. The encounter builder handles the maths, your VTT or table handles the space, and the game handles the rest.