diff --git a/README.md b/README.md
index 0bf7ba1..0198688 100644
--- a/README.md
+++ b/README.md
@@ -244,10 +244,11 @@ reproducible. A time-based id (UUID v7, ULID) therefore draws its timestamp from
 the rng, not the clock — the result is a valid, reproducible value, not a real
 point in time.
 
-There are three kinds. **Derivations** read the digits emitted so far, so put them
+There are four kinds. **Derivations** read the digits emitted so far, so put them
 after their payload; **generators** read only the rng, so they stand alone; one
-**session counter** (`seq`) advances state held on the faker. Arguments are
-validated at `New` (a bad count, range, or country fails fast).
+**session counter** (`seq`) advances state held on the faker; and one
+**computation** (`calc`) evaluates arithmetic over sibling fields. Arguments are
+validated at `New` (a bad count, range, country, or expression fails fast).
 
 | Function | Kind | Emits |
 |----------|------|-------|
@@ -264,6 +265,7 @@ validated at `New` (a bad count, range, or country fails fast).
 | `{float(min,max,dp)}` | generator | number in `[min, max]` with `dp` decimals |
 | `{iban(CC)}` | generator | a length- and mod-97-valid IBAN for country `CC` (BE, DE, DK, ES, FI, NO, SE) |
 | `{seq()}`, `{seq(name)}` | session counter | next integer (from 1) in this faker's sequence; `name` selects an independent counter |
+| `{calc(expr)}`, `{calc(expr,dp)}` | computation | value of an arithmetic expression over number literals and sibling fields; `dp` rounds |
 
 `{ean()}` is also the ISBN-13 check (an ISBN-13 *is* an EAN-13 — build the 978/979
 prefix in data and call `{ean()}`). `{iban()}` is a generator, not a derivation:
@@ -275,6 +277,23 @@ length and checksum, not real bank routing).
 and resets when you build a new faker — `seq` is reproducible by being ordered,
 not random. It's the natural fit for a primary-key column in the SQL example above.
 
+**Computation.** `{calc(expr)}` evaluates an arithmetic expression — `+ - * /`,
+parentheses and unary minus, the usual precedence — and emits the result. Operands
+are number literals and **sibling field names**, each rendered then read as a
+number; an optional second arg rounds to that many decimals (`{calc(net * qty, 2)}`),
+otherwise the value prints in minimal form. A hyphen is always subtraction, so a
+hyphenated field name can't be an operand. The expression is checked at `New`
+(parse, and that every name is a real field):
+
+```json
+{ "format": "{net} x {qty} = {calc(net * qty, 2)}", "net": ["19.99"], "qty": ["3"] }
+```
+
+renders `19.99 x 3 = 59.97`. Each name is rendered where it appears, so a field
+shown *and* used in a `calc` is drawn twice — keep a shared operand in a
+single-value field if the two must agree. A field that doesn't render to a number
+yields `NaN`, which prints rather than failing the render.
+
 **References.** A `{..path}` token renders a node from the **data root** instead
 of a sibling field — the dot path is the one `Fake` takes, resolved across every
 loaded directory. One category can borrow another, even across folders or layered
@@ -380,6 +399,7 @@ docker compose run --rm test                   # latest
 fakes.go        Fakes, New, options, seeding
 template.go     Fake, the recursive renderer (choices, format strings, paths)
 builtins.go     the {name()} function registry and its implementations
+calc.go         the {calc()} arithmetic evaluator: parser, eval, validation
 data.go         data loading: folders/files -> namespace tree, multi-path merge
 cmd/fakes/      the `fakes` CLI (New + Fake over stdout)
 data/           shipped data (JSON): locale folders + a misc folder
diff --git a/calc.go b/calc.go
new file mode 100644
index 0000000..3a115e3
--- /dev/null
+++ b/calc.go
@@ -0,0 +1,236 @@
+package fakes
+
+import (
+	"fmt"
+	"math"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// calc is the {calc(expr[, dp])} token: an arithmetic expression over number
+// literals and sibling-field names (rendered, then parsed as numbers), with
+// + - * /, unary minus and parentheses. Unlike a builtin it reads the field
+// environment, so expand and checkTokens route it here rather than the builtins
+// registry. The value prints in minimal decimal form, or rounded to dp decimals
+// when given. A field that doesn't render to a number becomes NaN, which
+// propagates and prints as "NaN" — visible, never a render error. The expression
+// is re-parsed each render, mirroring how expand re-scans the format; checkCalc
+// proves it parses (and names real fields) at New, so render never fails.
+
+// calcNode is a parsed expression node.
+type calcNode interface {
+	eval(s *session, fields map[string]node) float64
+}
+
+type calcNum float64 // a number literal
+type calcVar string  // a sibling-field name
+type calcNeg struct{ x calcNode }
+type calcBin struct { // a + - * / b
+	op   byte
+	l, r calcNode
+}
+
+func (n calcNum) eval(*session, map[string]node) float64 { return float64(n) }
+
+func (n calcVar) eval(s *session, fields map[string]node) float64 {
+	v, err := strconv.ParseFloat(strings.TrimSpace(render(s, fields[string(n)])), 64)
+	if err != nil {
+		return math.NaN() // a non-numeric operand stays visible, never an error
+	}
+	return v
+}
+
+func (n calcNeg) eval(s *session, fields map[string]node) float64 { return -n.x.eval(s, fields) }
+
+func (n calcBin) eval(s *session, fields map[string]node) float64 {
+	l, r := n.l.eval(s, fields), n.r.eval(s, fields)
+	switch n.op {
+	case '+':
+		return l + r
+	case '-':
+		return l - r
+	case '*':
+		return l * r
+	default: // '/'
+		return l / r
+	}
+}
+
+// checkCalc validates a calc token at compile time: a parseable expression whose
+// operands all name existing fields, and an optional non-negative integer dp.
+func checkCalc(args []string, fields map[string]node) error {
+	if len(args) < 1 || len(args) > 2 {
+		return fmt.Errorf("calc takes an expression and an optional decimals count, got %d args", len(args))
+	}
+	expr, err := parseCalc(args[0])
+	if err != nil {
+		return fmt.Errorf("calc(%q): %w", args[0], err)
+	}
+	for _, name := range calcVars(expr) {
+		if _, ok := fields[name]; !ok {
+			return fmt.Errorf("calc(%q): no field %q", args[0], name)
+		}
+	}
+	if len(args) == 2 {
+		if dp, err := strconv.Atoi(args[1]); err != nil || dp < 0 {
+			return fmt.Errorf("calc decimals %q must be a non-negative integer", args[1])
+		}
+	}
+	return nil
+}
+
+// calcEval renders a calc token. checkCalc proved the expression parses and the
+// decimals arg is valid, so neither step here can fail. dp -1 prints the minimal
+// form; a given dp rounds to that many places.
+func calcEval(s *session, args []string, fields map[string]node) string {
+	expr, _ := parseCalc(args[0])
+	dp := -1
+	if len(args) == 2 {
+		dp = atoi(args[1])
+	}
+	return strconv.FormatFloat(expr.eval(s, fields), 'f', dp, 64)
+}
+
+// calcVars lists the field names an expression references, for the existence
+// check in checkCalc.
+func calcVars(n calcNode) []string {
+	switch n := n.(type) {
+	case calcVar:
+		return []string{string(n)}
+	case calcNeg:
+		return calcVars(n.x)
+	case calcBin:
+		return append(calcVars(n.l), calcVars(n.r)...)
+	default:
+		return nil
+	}
+}
+
+// calcParser is a recursive-descent parser over the expression runes, threading
+// expr -> term -> factor for the standard * / before + - precedence.
+type calcParser struct {
+	rs  []rune
+	pos int
+}
+
+// parseCalc parses a whole expression, requiring it to consume all input.
+func parseCalc(expr string) (calcNode, error) {
+	p := &calcParser{rs: []rune(expr)}
+	if p.space(); p.pos >= len(p.rs) {
+		return nil, fmt.Errorf("empty expression")
+	}
+	n, err := p.expr()
+	if err != nil {
+		return nil, err
+	}
+	if p.space(); p.pos != len(p.rs) {
+		return nil, fmt.Errorf("unexpected %q", string(p.rs[p.pos:]))
+	}
+	return n, nil
+}
+
+func (p *calcParser) space() {
+	for p.pos < len(p.rs) && unicode.IsSpace(p.rs[p.pos]) {
+		p.pos++
+	}
+}
+
+func (p *calcParser) expr() (calcNode, error) { return p.binary(p.term, '+', '-') }
+func (p *calcParser) term() (calcNode, error) { return p.binary(p.factor, '*', '/') }
+
+// binary parses a left-associative run of next() operands joined by the given
+// operators, the one shape expr and term share.
+func (p *calcParser) binary(next func() (calcNode, error), ops ...byte) (calcNode, error) {
+	n, err := next()
+	if err != nil {
+		return nil, err
+	}
+	for {
+		p.space()
+		if p.pos >= len(p.rs) || !contains(ops, byte(p.rs[p.pos])) {
+			return n, nil
+		}
+		op := byte(p.rs[p.pos])
+		p.pos++
+		r, err := next()
+		if err != nil {
+			return nil, err
+		}
+		n = calcBin{op, n, r}
+	}
+}
+
+func (p *calcParser) factor() (calcNode, error) {
+	p.space()
+	if p.pos >= len(p.rs) {
+		return nil, fmt.Errorf("unexpected end of expression")
+	}
+	switch c := p.rs[p.pos]; {
+	case c == '-':
+		p.pos++
+		x, err := p.factor()
+		if err != nil {
+			return nil, err
+		}
+		return calcNeg{x}, nil
+	case c == '(':
+		p.pos++
+		n, err := p.expr()
+		if err != nil {
+			return nil, err
+		}
+		if p.space(); p.pos >= len(p.rs) || p.rs[p.pos] != ')' {
+			return nil, fmt.Errorf("missing ')'")
+		}
+		p.pos++
+		return n, nil
+	case c == '.' || c >= '0' && c <= '9':
+		return p.number()
+	case c == '_' || unicode.IsLetter(c):
+		return p.ident()
+	default:
+		return nil, fmt.Errorf("unexpected %q", string(c))
+	}
+}
+
+func (p *calcParser) number() (calcNode, error) {
+	start, dot := p.pos, false
+	for p.pos < len(p.rs) {
+		if c := p.rs[p.pos]; c >= '0' && c <= '9' {
+			p.pos++
+		} else if c == '.' && !dot {
+			dot, p.pos = true, p.pos+1
+		} else {
+			break
+		}
+	}
+	v, err := strconv.ParseFloat(string(p.rs[start:p.pos]), 64)
+	if err != nil {
+		return nil, fmt.Errorf("bad number %q", string(p.rs[start:p.pos]))
+	}
+	return calcNum(v), nil
+}
+
+// ident reads a field name: a letter or '_', then letters, digits or '_'. A '-'
+// is always the minus operator, so a hyphenated field name can't be an operand.
+func (p *calcParser) ident() (calcNode, error) {
+	start := p.pos
+	for p.pos < len(p.rs) {
+		if c := p.rs[p.pos]; c == '_' || unicode.IsLetter(c) || unicode.IsDigit(c) {
+			p.pos++
+		} else {
+			break
+		}
+	}
+	return calcVar(string(p.rs[start:p.pos])), nil
+}
+
+func contains(bs []byte, b byte) bool {
+	for _, x := range bs {
+		if x == b {
+			return true
+		}
+	}
+	return false
+}
diff --git a/calc_test.go b/calc_test.go
new file mode 100644
index 0000000..f62d48e
--- /dev/null
+++ b/calc_test.go
@@ -0,0 +1,84 @@
+package fakes
+
+import "testing"
+
+// TestCalcArithmetic pins the operators, precedence, parentheses and unary minus
+// over number literals.
+func TestCalcArithmetic(t *testing.T) {
+	f := engine(1)
+	cases := map[string]string{
+		`{"format":"{calc(2 + 3)}"}`:       "5",
+		`{"format":"{calc(2 * 3 + 4)}"}`:   "10", // * binds tighter than +
+		`{"format":"{calc(2 + 3 * 4)}"}`:   "14",
+		`{"format":"{calc((2 + 3) * 4)}"}`: "20", // parentheses override
+		`{"format":"{calc(10 / 4)}"}`:      "2.5",
+		`{"format":"{calc(-2 + 5)}"}`:      "3", // unary minus
+		`{"format":"{calc(2 - -3)}"}`:      "5",
+		`{"format":"{calc(1.5 * 2)}"}`:     "3", // whole result drops the decimals
+	}
+	for tmpl, want := range cases {
+		if got := mustRender(t, f, tmpl); got != want {
+			t.Errorf("%s = %q, want %q", tmpl, got, want)
+		}
+	}
+}
+
+// TestCalcAuto pins the default (no-dp) rendering: minimal decimal form, no
+// scientific notation, whole numbers without a fraction.
+func TestCalcAuto(t *testing.T) {
+	f := engine(1)
+	cases := map[string]string{
+		`{"format":"{calc(10 / 3)}"}`: "3.3333333333333335",
+		`{"format":"{calc(6 / 2)}"}`:  "3",
+		`{"format":"{calc(1 / 4)}"}`:  "0.25",
+	}
+	for tmpl, want := range cases {
+		if got := mustRender(t, f, tmpl); got != want {
+			t.Errorf("%s = %q, want %q", tmpl, got, want)
+		}
+	}
+}
+
+// TestCalcDecimals pins the optional decimals arg: rounds to dp places, dp 0
+// drops the fraction.
+func TestCalcDecimals(t *testing.T) {
+	f := engine(1)
+	cases := map[string]string{
+		`{"format":"{calc(10 / 3, 2)}"}`: "3.33",
+		`{"format":"{calc(10 / 3, 0)}"}`: "3",
+		`{"format":"{calc(2 * 3, 2)}"}`:  "6.00",
+	}
+	for tmpl, want := range cases {
+		if got := mustRender(t, f, tmpl); got != want {
+			t.Errorf("%s = %q, want %q", tmpl, got, want)
+		}
+	}
+}
+
+// TestCalcFields pins that bare names resolve to sibling fields, rendered then
+// parsed as numbers.
+func TestCalcFields(t *testing.T) {
+	if got := mustRender(t, engine(1), `{"format":"{calc(price * qty, 2)}","price":["19.99"],"qty":["3"]}`); got != "59.97" {
+		t.Fatalf("calc over fields = %q, want 59.97", got)
+	}
+	// A field that is itself a template renders before parsing.
+	if got := mustRender(t, engine(1), `{"format":"{calc(a - b)}","a":[{"format":"{n}","n":["10"]}],"b":["3"]}`); got != "7" {
+		t.Fatalf("calc(a - b) = %q, want 7", got)
+	}
+}
+
+// TestCalcNonNumericIsNaN pins the never-fail rule: a field that doesn't render
+// to a number becomes NaN, which propagates and prints visibly.
+func TestCalcNonNumericIsNaN(t *testing.T) {
+	if got := mustRender(t, engine(1), `{"format":"{calc(x * 2)}","x":["abc"]}`); got != "NaN" {
+		t.Fatalf("calc over non-numeric field = %q, want NaN", got)
+	}
+}
+
+// TestCalcReproducible pins that a calc over a random operand stays seed-stable.
+func TestCalcReproducible(t *testing.T) {
+	tmpl := `{"format":"{calc(q * 2 + 1)}","q":[{"format":"{int(1,1000000)}"}]}`
+	if a, b := mustRender(t, engine(7), tmpl), mustRender(t, engine(7), tmpl); a != b {
+		t.Fatalf("calc not reproducible: %q != %q", a, b)
+	}
+}
diff --git a/template.go b/template.go
index ed608b1..fa59c96 100644
--- a/template.go
+++ b/template.go
@@ -163,7 +163,11 @@ func expand(s *session, format string, fields map[string]node) string {
 			}
 			body := string(rs[i+1 : end])
 			if name, args, ok := funcCall(body); ok {
-				b.WriteString(builtins[name].call(s, b.String(), args)) // b.String() is the output so far
+				if name == "calc" { // calc reads sibling fields, not just (rng, emitted)
+					b.WriteString(calcEval(s, args, fields))
+				} else {
+					b.WriteString(builtins[name].call(s, b.String(), args)) // b.String() is the output so far
+				}
 			} else {
 				b.WriteString(resolve(s, body, fields))
 			}
@@ -342,7 +346,11 @@ func checkTokens(format string, fields map[string]node) error {
 			}
 			body := string(rs[i+1 : end])
 			if strings.IndexByte(body, '(') >= 0 { // a function token, not a field
-				if err := checkFunc(body); err != nil {
+				if name, args, ok := funcCall(body); ok && name == "calc" {
+					if err := checkCalc(args, fields); err != nil {
+						return fmt.Errorf("token {%s}: %w", body, err)
+					}
+				} else if err := checkFunc(body); err != nil {
 					return err
 				}
 			} else {
diff --git a/template_test.go b/template_test.go
index 159cae3..f356af3 100644
--- a/template_test.go
+++ b/template_test.go
@@ -193,6 +193,14 @@ func TestCompileErrors(t *testing.T) {
 		`{"format":"{float(1,2,-1)}"}`,                                  // negative decimals
 		`{"format":"{iban(US)}"}`,                                       // unsupported country
 		`{"format":"{seq(a,b)}"}`,                                       // seq takes at most one name
+		`{"format":"{calc()}"}`,                                         // calc needs an expression
+		`{"format":"{calc(1 +)}"}`,                                      // dangling operator
+		`{"format":"{calc((1 + 2)}"}`,                                   // unbalanced parenthesis
+		`{"format":"{calc(1 2)}"}`,                                      // two operands, no operator
+		`{"format":"{calc(price)}"}`,                                    // operand names no field
+		`{"format":"{calc(1, 2, 3)}"}`,                                  // too many args
+		`{"format":"{calc(1, x)}"}`,                                     // decimals arg not an integer
+		`{"format":"{calc(1, -1)}"}`,                                    // decimals negative
 	} {
 		if _, err := compile(parse(t, bad)); err == nil {
 			t.Errorf("compile(%s) = nil error, want error", bad)