diff --git a/README b/README
index d4dced0..876ebd7 100644
--- a/README
+++ b/README
@@ -509,7 +509,7 @@ Expressions
 
 Expressions can contain numeric constants (in non-exponential notation),
 variable names, the arithmetic operations +, -, *, /, unary -, and the
-functions sin(), cos(), and sqrt().
+functions sin(), cos(), sqrt(), and floor().
 
 Parentheses can be used to change precedence.
 
@@ -520,6 +520,10 @@ The argument of sqrt() can be dimensionless or have a dimension with an
 exponent that's a multiple of two. E.g., sqrt(2) and sqrt(2mm*3mm) are
 valid expressions, sqrt(2mm) isn't.
 
+The function floor() returns the next integer that is below or equal to
+the argument. If the argument has a dimension, that dimension is
+preserved. E.g., floor(-1.2) returns -2, floor(4.7mm) returns 4mm.
+
 
 GUI
 ---
diff --git a/expr.c b/expr.c
index 66170b1..c1753c4 100644
--- a/expr.c
+++ b/expr.c
@@ -322,6 +322,17 @@ struct num op_minus(const struct expr *self, const struct frame *frame)
 }
 
 
+struct num op_floor(const struct expr *self, const struct frame *frame)
+{
+	struct num res;
+
+	res = eval_num(self->u.op.a, frame);
+	if (!is_undef(res))
+		res.n = floor(res.n);
+	return res;
+}
+
+
 #define	BINARY						\
 	struct num a, b, res;				\
 							\
@@ -533,7 +544,7 @@ static void vacate_op(struct expr *expr)
 		free(expr->u.str);
 		return;
 	}
-	if (expr->op == op_minus ||
+	if (expr->op == op_minus || expr->op == op_floor ||
 	    expr->op == op_sin || expr->op == op_cos || expr->op == op_sqrt) {
 		free_expr(expr->u.op.a);
 		return;
diff --git a/expr.h b/expr.h
index 6e6c201..f89f210 100644
--- a/expr.h
+++ b/expr.h
@@ -1,8 +1,8 @@
 /*
  * expr.h - Expressions and values
  *
- * Written 2009 by Werner Almesberger
- * Copyright 2009 by Werner Almesberger
+ * Written 2009, 2012 by Werner Almesberger
+ * Copyright 2009, 2012 by Werner Almesberger
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -116,6 +116,7 @@ struct num op_cos(const struct expr *self, const struct frame *frame);
 struct num op_sqrt(const struct expr *self, const struct frame *frame);
 
 struct num op_minus(const struct expr *self, const struct frame *frame);
+struct num op_floor(const struct expr *self, const struct frame *frame);
 
 struct num op_add(const struct expr *self, const struct frame *frame);
 struct num op_sub(const struct expr *self, const struct frame *frame);
diff --git a/fpd.y b/fpd.y
index 06278e0..d1db83c 100644
--- a/fpd.y
+++ b/fpd.y
@@ -48,7 +48,7 @@ static struct obj **next_obj;
 
 static int n_vars, n_values;
 
-static const char *id_sin, *id_cos, *id_sqrt;
+static const char *id_sin, *id_cos, *id_sqrt, *id_floor;
 
 static struct tsort *tsort;
 
@@ -491,6 +491,7 @@ all:
 			id_sin = unique("sin");
 			id_cos = unique("cos");
 			id_sqrt = unique("sqrt");
+			id_floor = unique("floor");
 		}
 	    fpd
 	| START_EXPR expr
@@ -1245,6 +1246,8 @@ primary_expr:
 				$$ = binary_op(op_cos, $3, NULL);
 			else if ($1 == id_sqrt)
 				$$ = binary_op(op_sqrt, $3, NULL);
+			else if ($1 == id_floor)
+				$$ = binary_op(op_floor, $3, NULL);
 			else {
 				yyerrorf("unknown function \"%s\"", $1);
 				YYABORT;
diff --git a/test/floor b/test/floor
new file mode 100755
index 0000000..ced963e
--- /dev/null
+++ b/test/floor
@@ -0,0 +1,22 @@
+#!/bin/sh
+. ./Common
+
+###############################################################################
+
+fped "floor: 4.7mm" <<EOF
+%print floor(4.7mm)
+EOF
+expect <<EOF
+4mm
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "floor: -1.2m" <<EOF
+%print floor(-1.2)
+EOF
+expect <<EOF
+-2
+EOF
+
+###############################################################################
diff --git a/unparse.c b/unparse.c
index 582cdeb..92bc28a 100644
--- a/unparse.c
+++ b/unparse.c
@@ -1,8 +1,8 @@
 /*
  * unparse.c - Dump an expression tree into a string
  *
- * Written 2009 by Werner Almesberger
- * Copyright 2009 by Werner Almesberger
+ * Written 2009, 2012 by Werner Almesberger
+ * Copyright 2009, 2012 by Werner Almesberger
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -39,7 +39,7 @@ static int precedence(op_type op)
 	if (op == op_minus)
 		return prec_unary;
 	if (op == op_num || op == op_string || op == op_var ||
-	    op == op_sin || op == op_cos || op == op_sqrt)
+	    op == op_sin || op == op_cos || op == op_sqrt || op == op_floor)
 		return prec_primary;
 	abort();
 }
@@ -125,6 +125,8 @@ static char *unparse_op(const struct expr *expr, enum prec prec)
 		return unparse_fn("cos", expr);
 	if (expr->op == op_sqrt)
 		return unparse_fn("sqrt", expr);
+	if (expr->op == op_floor)
+		return unparse_fn("floor", expr);
 	abort();
 }