diff --git a/README.md b/README.md
index 8fcd82d..5309269 100644
--- a/README.md
+++ b/README.md
@@ -460,6 +460,43 @@ fmt.Println(poh) // 11
 
 The package favors correctness and simplicity first, while keeping the implementation small and allocation-free in normal use.
 
+## Optional Parsing
+
+The core package does not parse unit names. Import `parse` when string input is
+part of an application's boundary:
+
+```go
+length, err := parse.ParseLength("5 km")
+mass, err := parse.ParseMass("10lb")
+_ = length.To(quant.Meter) // 5000
+_ = mass.To(quant.Kilogram) // 4.5359237
+```
+
+Custom dimensions can provide their own accepted symbols with `parse.UnitSet`:
+
+```go
+type Widget struct{}
+type Box struct{}
+
+func (Box) ToBase(v float64) float64   { return v * 12 }
+func (Box) FromBase(v float64) float64 { return v / 12 }
+func (Box) Dimension() Widget          { return Widget{} }
+
+widgets, err := parse.Parse("2 box", parse.UnitSet[Widget]{"box": Box{}})
+```
+
+## CODATA Constants
+
+`codata` contains selected 2022 CODATA recommended values from NIST. Values
+are plain `float64` constants with their SI units stated in the package docs;
+uncertainties and correlations are intentionally left to the authoritative
+CODATA tables.
+
+```go
+speedOfLight := codata.SpeedOfLight // m/s
+elementaryCharge := codata.ElementaryCharge // C
+```
+
 ## Development
 
 Run tests with:
diff --git a/codata/constants.go b/codata/constants.go
new file mode 100644
index 0000000..ef07277
--- /dev/null
+++ b/codata/constants.go
@@ -0,0 +1,34 @@
+// Package codata contains selected 2022 CODATA recommended constants.
+//
+// Values are exposed as float64 constants in the SI units documented by NIST.
+// Uncertainties are not represented; consult the source when uncertainty or
+// correlations matter. This package intentionally does not invent Quantity
+// dimensions for compound units that are not modeled by quant.
+package codata
+
+const (
+	// SpeedOfLight is the speed of light in vacuum, in m/s (exact).
+	SpeedOfLight = 299792458.0
+	// PlanckConstant is Planck's constant, in J s (exact).
+	PlanckConstant = 6.62607015e-34
+	// ReducedPlanckConstant is the reduced Planck constant, in J s.
+	ReducedPlanckConstant = 1.054571817e-34
+	// ElementaryCharge is the elementary charge, in C (exact).
+	ElementaryCharge = 1.602176634e-19
+	// BoltzmannConstant is the Boltzmann constant, in J/K (exact).
+	BoltzmannConstant = 1.380649e-23
+	// AvogadroConstant is the Avogadro constant, in 1/mol (exact).
+	AvogadroConstant = 6.02214076e23
+	// GravitationalConstant is Newton's constant of gravitation, in m^3 kg^-1 s^-2.
+	GravitationalConstant = 6.67430e-11
+	// FineStructureConstant is the fine-structure constant, dimensionless.
+	FineStructureConstant = 7.2973525643e-3
+	// ElectronMass is the electron mass, in kg.
+	ElectronMass = 9.1093837139e-31
+	// ProtonMass is the proton mass, in kg.
+	ProtonMass = 1.67262192595e-27
+	// MolarGasConstant is the molar gas constant, in J/(mol K).
+	MolarGasConstant = 8.314462618
+	// VacuumPermittivity is the vacuum electric permittivity, in F/m.
+	VacuumPermittivity = 8.8541878188e-12
+)
diff --git a/codata/constants_test.go b/codata/constants_test.go
new file mode 100644
index 0000000..5bb98f2
--- /dev/null
+++ b/codata/constants_test.go
@@ -0,0 +1,19 @@
+package codata_test
+
+import (
+	"testing"
+
+	"github.com/nodivbyzero/quant/codata"
+)
+
+func TestDefiningConstants(t *testing.T) {
+	if codata.SpeedOfLight != 299792458 {
+		t.Fatalf("speed of light changed: got %g", codata.SpeedOfLight)
+	}
+	if codata.ElementaryCharge != 1.602176634e-19 {
+		t.Fatalf("elementary charge changed: got %g", codata.ElementaryCharge)
+	}
+	if codata.AvogadroConstant != 6.02214076e23 {
+		t.Fatalf("Avogadro constant changed: got %g", codata.AvogadroConstant)
+	}
+}
diff --git a/parse/parse.go b/parse/parse.go
new file mode 100644
index 0000000..771b125
--- /dev/null
+++ b/parse/parse.go
@@ -0,0 +1,122 @@
+// Package parse provides optional string parsing for quant quantities.
+//
+// The core quant package deliberately has no unit-name parser. Callers opt in
+// here and provide the unit registry they want to accept.
+package parse
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/nodivbyzero/quant"
+)
+
+// UnitSet maps accepted symbols to units for one dimension.
+type UnitSet[D quant.Dimension] map[string]quant.Unit[D]
+
+// Parse parses a scalar followed by an optional unit symbol. Both "5 km" and
+// "5km" are accepted. A missing symbol uses the empty-string entry in units.
+func Parse[D quant.Dimension](input string, units UnitSet[D]) (quant.Quantity[D], error) {
+	value, symbol, err := split(input)
+	if err != nil {
+		return quant.Quantity[D]{}, err
+	}
+
+	u, ok := units[symbol]
+	if !ok {
+		return quant.Quantity[D]{}, fmt.Errorf("parse: unknown unit %q", symbol)
+	}
+
+	return quant.New[D](value, u), nil
+}
+
+// ParseLength parses a length using the built-in length symbols in LengthUnits.
+func ParseLength(input string) (quant.Quantity[quant.Length], error) {
+	return Parse(input, LengthUnits)
+}
+
+// ParseMass parses a mass using the built-in mass symbols in MassUnits.
+func ParseMass(input string) (quant.Quantity[quant.Mass], error) {
+	return Parse(input, MassUnits)
+}
+
+// ParseTime parses a time using the built-in time symbols in TimeUnits.
+func ParseTime(input string) (quant.Quantity[quant.Time], error) {
+	return Parse(input, TimeUnits)
+}
+
+// ParseTemperature parses a temperature using the built-in temperature symbols
+// in TemperatureUnits.
+func ParseTemperature(input string) (quant.Quantity[quant.Temperature], error) {
+	return Parse(input, TemperatureUnits)
+}
+
+var LengthUnits = UnitSet[quant.Length]{
+	"": quant.Meter, "m": quant.Meter, "km": quant.Kilometer, "cm": quant.Centimeter,
+	"mm": quant.Millimeter, "mi": quant.Mile, "ft": quant.Foot, "in": quant.Inch,
+}
+
+var MassUnits = UnitSet[quant.Mass]{
+	"": quant.Kilogram, "kg": quant.Kilogram, "g": quant.Gram, "mg": quant.Milligram,
+	"lb": quant.Pound, "oz": quant.Ounce,
+}
+
+var TimeUnits = UnitSet[quant.Time]{
+	"": quant.Second, "s": quant.Second, "ms": quant.Millisecond, "min": quant.Minute,
+	"h": quant.Hour, "d": quant.Day,
+}
+
+var TemperatureUnits = UnitSet[quant.Temperature]{
+	"": quant.Kelvin, "K": quant.Kelvin, "C": quant.Celsius, "F": quant.Fahrenheit,
+}
+
+func split(input string) (float64, string, error) {
+	s := strings.TrimSpace(input)
+	if s == "" {
+		return 0, "", fmt.Errorf("parse: empty input")
+	}
+
+	end := 0
+	if s[0] == '+' || s[0] == '-' {
+		end++
+	}
+	digits := false
+	for end < len(s) && s[end] >= '0' && s[end] <= '9' {
+		end++
+		digits = true
+	}
+	if end < len(s) && s[end] == '.' {
+		end++
+		for end < len(s) && s[end] >= '0' && s[end] <= '9' {
+			end++
+			digits = true
+		}
+	}
+	if !digits {
+		return 0, "", fmt.Errorf("parse: invalid number in %q", input)
+	}
+	if end < len(s) && (s[end] == 'e' || s[end] == 'E') {
+		end++
+		if end < len(s) && (s[end] == '+' || s[end] == '-') {
+			end++
+		}
+		expStart := end
+		for end < len(s) && s[end] >= '0' && s[end] <= '9' {
+			end++
+		}
+		if expStart == end {
+			return 0, "", fmt.Errorf("parse: invalid exponent in %q", input)
+		}
+	}
+
+	value, err := strconv.ParseFloat(s[:end], 64)
+	if err != nil {
+		return 0, "", fmt.Errorf("parse: %w", err)
+	}
+	symbol := strings.TrimSpace(s[end:])
+	if strings.ContainsAny(symbol, " \t\r\n") {
+		return 0, "", fmt.Errorf("parse: invalid unit %q", symbol)
+	}
+	return value, symbol, nil
+}
diff --git a/parse/parse_test.go b/parse/parse_test.go
new file mode 100644
index 0000000..841ac7d
--- /dev/null
+++ b/parse/parse_test.go
@@ -0,0 +1,43 @@
+package parse_test
+
+import (
+	"math"
+	"testing"
+
+	"github.com/nodivbyzero/quant"
+	"github.com/nodivbyzero/quant/parse"
+)
+
+func TestBuiltInParsers(t *testing.T) {
+	length, err := parse.ParseLength("5 km")
+	if err != nil || length.To(quant.Meter) != 5000 {
+		t.Fatalf("length parse: value=%v err=%v", length.To(quant.Meter), err)
+	}
+
+	mass, err := parse.ParseMass("10lb")
+	if err != nil || math.Abs(mass.To(quant.Kilogram)-4.5359237) > 1e-12 {
+		t.Fatalf("mass parse: value=%v err=%v", mass.To(quant.Kilogram), err)
+	}
+
+	temperature, err := parse.ParseTemperature("32 F")
+	if err != nil || math.Abs(temperature.To(quant.Kelvin)-273.15) > 1e-12 {
+		t.Fatalf("temperature parse: value=%v err=%v", temperature.To(quant.Kelvin), err)
+	}
+}
+
+func TestCustomRegistry(t *testing.T) {
+	units := parse.UnitSet[Widget]{
+		"box": BoxUnit{},
+	}
+	q, err := parse.Parse("2 box", units)
+	if err != nil || q.To(BoxUnit{}) != 2 {
+		t.Fatalf("custom parse: value=%v err=%v", q.To(BoxUnit{}), err)
+	}
+}
+
+type Widget struct{}
+type BoxUnit struct{}
+
+func (BoxUnit) ToBase(v float64) float64   { return v * 12 }
+func (BoxUnit) FromBase(v float64) float64 { return v / 12 }
+func (BoxUnit) Dimension() Widget          { return Widget{} }