diff --git a/components/app.js b/components/app.js
index 4aa4194..3ecf1b5 100644
--- a/components/app.js
+++ b/components/app.js
@@ -1928,8 +1928,12 @@ export default class App extends Component {
 		}
 
 		let commandOnly = false;
+		let privmsgMaxLen;
 		if (activeBuffer && activeBuffer.type === BufferType.SERVER) {
 			commandOnly = true;
+		} else if (activeBuffer) {
+			let client = this.clients.get(activeBuffer.server);
+			privmsgMaxLen = irc.getMaxPrivmsgLen(client.isupport, client.nick, activeBuffer.name);
 		}
 
 		let app = html`
@@ -1981,6 +1985,7 @@ export default class App extends Component {
 				onSubmit=${this.handleComposerSubmit}
 				autocomplete=${this.autocomplete}
 				commandOnly=${commandOnly}
+				maxLen=${privmsgMaxLen}
 			/>
 			${dialog}
 			${error}
diff --git a/components/composer.js b/components/composer.js
index 6938f13..28fe664 100644
--- a/components/composer.js
+++ b/components/composer.js
@@ -222,6 +222,7 @@ export default class Composer extends Component {
 					placeholder=${placeholder}
 					enterkeyhint="send"
 					onKeyDown=${this.handleInputKeyDown}
+					maxlength=${this.props.maxLen}
 				/>
 			</form>
 		`;
diff --git a/lib/irc.js b/lib/irc.js
index fe8de45..03ceb8d 100644
--- a/lib/irc.js
+++ b/lib/irc.js
@@ -479,6 +479,36 @@ export class Isupport {
 	bot() {
 		return this.raw.get("BOT");
 	}
+
+	userLen() {
+		if (!this.raw.has("USERLEN")) {
+			return 20;
+		}
+		return parseInt(this.raw.get("USERLEN"), 10);
+	}
+
+	hostLen() {
+		if (!this.raw.has("HOSTLEN")) {
+			return 63;
+		}
+		return parseInt(this.raw.get("HOSTLEN"), 10);
+	}
+
+	lineLen() {
+		if (!this.raw.has("LINELEN")) {
+			return 512;
+		}
+		return parseInt(this.raw.get("LINELEN"), 10);
+	}
+}
+
+export function getMaxPrivmsgLen(isupport, nick, target) {
+	let user = "_".repeat(isupport.userLen());
+	let host = "_".repeat(isupport.hostLen());
+	let prefix = { name: nick, user, host };
+	let msg = { prefix, command: "PRIVMSG", params: [target, ""] };
+	let raw = formatMessage(msg) + "\r\n";
+	return isupport.lineLen() - raw.length;
 }
 
 export const CaseMapping = {