How do I measure the lumens of a flashlight?

How can I measure how many lumens my flashlight has? is there some kind of meter capable of measuring lumens?

"Lumens", no. Not with a simple meter. It requires a spherical integration meter. These are...rather expensive, and not common.

Labsphere : Light Measurement Spheres

Complete systems for this, or handheld units, you're talking $3000 and up.

Candlepower/candela/foot-candles on the other hand is easier, but not really nearly as telling or comparable across different lights.

So how do I know if the flash light I built is really 1000 lumens and not less?

If you know the LED and the current you can compare to this list.

Quick reference list of common powerleds brightness bins at Tj=25C
Actual brightness will be around 10% less due to heating of the led.


Cree XR-E, XP-E & XP-G brightness at 350mA

P4 80.6-87.4 lm @ 350mA
Q2 87.4-93.9 lm @ 350mA
Q3 93.9-100 lm @ 350mA
Q4 100-107 lm @ 350mA
Q5 107-114 lm @ 350mA
R2 114-122 lm @ 350mA
R3 122-130 lm @ 350mA
R4 130-139 lm @ 350mA
R5 139-148 lm @ 350mA
S2 148-156 lm @ 350mA
S3 156-164 lm @ 350mA
S4 164-172 lm @ 350mA


Cree XR-E and XP-E brightness at 1A

P4 177-192 lm @ 1A
Q2 192-207 lm @ 1A
Q3 207-220 lm @ 1A
Q4 220-233 lm @ 1A
Q5 233-251 lm @ 1A
R2 251-268 lm @ 1A
R3 268-286 lm @ 1A*
R4 286-306 lm @ 1A


Cree XP-G brightness at 1A

R2 285-305 lm @ 1A
R3 305-325 lm @ 1A
R4 325-348 lm @ 1A
R5 348-370 lm @ 1A
S2 370-390 lm @ 1A
S3 390-410 lm @ 1A*
S4 410-430 lm @ 1A


Cree XP-G brightness at 1.5A

R2 380-406 lm @ 1.5A
R3 406-433 lm @ 1.5A
R4 433-463 lm @ 1.5A
R5 463-493 lm @ 1.5A
S2 493-520 lm @ 1.5A
S3 520-547 lm @ 1.5A*
S4 547-573 lm @ 1.5A

Cree XM-L brightness at 700mA

T5 260-280 lm @ 700mA
T6 280-300 lm @ 700mA*
U2 300-320 lm @ 700mA
U3 320-340 lm @ 700mA


Cree XM-L brightness at 1.5A

T5 504-543 lm @ 1.5A
T6 543-582 lm @ 1.5A*
U2 582-621 lm @ 1.5A
U3 621-660 lm @ 1.5A


Cree XM-L brightness at 3.oA

T5 845-910 lm @ 3.0A
T6 910-975 lm @ 3.0A*
U2 975-1040 lm @ 3.0A
U3 1040-1105 lm @ 3.0A


Cree MC-E brightness at 350mA per die, 1.4A total current

H 280-320 lm @ 350mA (1.4A)
J 320-370 lm @ 350mA (1.4A)
K 370-430 lm @ 350mA (1.4A)
M 430-490 lm @ 350mA (1.4A)
N 490-550 lm @ 350mA (1.4A)*
P 550-620 lm @ 350mA (1.4A)


MC-E brightness at 700mA per die, 2.8A total current

H 490-560 lm @ 700mA (2.8A)
J 560-648 lm @ 700mA (2.8A)
K 648-753 lm @ 700mA (2.8A)
M 753-858 lm @ 700mA (2.8A)
N 858-962 lm @ 700mA (2.8A)*
P 962-1085 lm @ 700mA (2.8A)


Luminus SST-50 brightness at 1.75A, 350mA per mm²

F 275-350 lm @ 1.75A (350mA/mm²)
G 350-425 lm @ 1.75A (350mA/mm²)
H 425-500 lm @ 1.75A (350mA/mm²)
J 500-600 lm @ 1.75A (350mA/mm²)*
K 600-700 lm @ 1.75A (350mA/mm²)


Luminus SST-50 brightness at 2.8A, 560mA per mm²

F 454-576 lm @ 2.8A (560mA/mm²)
G 576-701 lm @ 2.8A (560mA/mm²)
H 701-825 lm @ 2.8A (560mA/mm²)
J 825-990 lm @ 2.8A (560mA/mm²)*
K 990-1155 lm @ 2.8A (560mA/mm²)


Luminus SST-50 brightness at 5A, 1A per mm²

F 743-945 lm @ 5A (1A/mm²)
G 945-1148 lm @ 5A (1A/mm²)
H 1148-1350 lm @ 5A (1A/mm²)
J 1350-1620 lm @ 5A (1A/mm²)*
K 1620-1890 lm @ 5A (1A/mm²)


Luminus SST-90 brightness at 3.15A, 350mA per mm²

K 600-700 lm @ 3.15A (350mA/mm²)
L 700-850 lm @ 3.15A (350mA/mm²)
M 850-1000 lm @ 3.15A (350mA/mm²)
N 1000-1200 lm @ 3.15A (350mA/mm²)*
P 1200-1450 lm @ 3.15A (350mA/mm²)


Luminus SST-90 brightness at 5A, 560mA per mm²

K 930-1085 lm @ 5A (560mA/mm²)
L 1085-1318 lm @ 5A (560mA/mm²)
M 1318-1550 lm @ 5A (560mA/mm²)
N 1550-1860 lm @ 5A (560mA/mm²)*
P 1860-2248 lm @ 5A (560mA/mm²)


Luminus SST-90 brightness at 9A, 1A per mm²

K 1620-1890 lm @ 9A (1A/mm²)
L 1890-2295 lm @ 9A (1A/mm²)
M 2295-2700 lm @ 9A (1A/mm²)
N 2700-3240 lm @ 9A (1A/mm²)*
P 3240-3915 lm @ 9A (1A/mm²)


Seoul SemiConductor P4 brightness at 350mA

S1 54-60 lm @ 350mA
S2 60-70 lm @ 350mA
T1 70-80 lm @ 350mA
T2 80-91 lm @ 350mA
U1 91-100 lm @ 350mA
U2 100-109 lm @ 350mA*
U3 109-119 lm @ 350mA


Seoul SemiConductor P4 brightness at 1A

S1 127-141 lm @ 1A
S2 141-165 lm @ 1A
T1 165-188 lm @ 1A
T2 188-214 lm @ 1A
U1 214-235 lm @ 1A
U2 235-256 lm @ 1A*
U3 256-280 lm @ 1A


Seoul SemiConductor P7 brightness at 1.4A total current, 350mA per die

A 251-326 lm @ 1.4A (350mA)
B 326-400 lm @ 1.4A (350mA)
C 400-457 lm @ 1.4A (350mA)
D 457-514 lm @ 1.4A (350mA)*
E 514-629 lm @ 1.4A (350mA)


Seoul SemiConductor P7 brightness at 2.8A total current, 700mA per die

A 440-570 lm @ 2.8A (700mA)
B 570-700 lm @ 2.8A (700mA)
C 700-800 lm @ 2.8A (700mA)
D 800-900 lm @ 2.8A (700mA)*
E 900-1100 lm @ 2.8A (700mA)


Lumileds Luxeon Rebel brightness at 350mA

040 40+ lm @ 350mA
050 50+ lm @ 350mA
060 60+ lm @ 350mA
070 70+ lm @ 350mA
080 80+ lm @ 350mA
090 90+ lm @ 350mA
100 100+ lm @ 350mA*
110 110+ lm @ 350mA


Lumileds Luxeon Rebel brightness at 1A

040 84+ lm @ 1A
050 105+ lm @ 1A
060 126+ lm @ 1A
070 147+ lm @ 1A
080 168+ lm @ 1A
090 189+ lm @ 1A
100 210+ lm @ 1A*
110 231+ lm @ 1A


*(maximum available flux bin as of writing)

Sources:
CREE | Lighting LEDs Documentation
http://www.luminus.com/content1454
Seoul Semiconductor, Inc
LUXEON Rebel White LEDs | Philips Lumileds Lighting Company

Don't know how well they compare but people do build DIY integrating spheres. You need a good coating. Improving the DIY integrating sphere

My LED is a XM-L-H it suppose to be a 1000 lumen LED @ 3000mA.