how to calculate a net's FLOPs in CNN

For online tool see http://dgschwend.github.io/netscope/#/editor . For alexnet see http://dgschwend.github.io/netscope/#/preset/alexnet . This supports most wide known layers. For custom layers you will have to calculate yourself.

For future visitors, if you use Keras and TensorFlow as Backend then you can try the following example. It calculates the FLOPs for the MobileNet.

import tensorflow as tf
import keras.backend as K
from keras.applications.mobilenet import MobileNet

run_meta = tf.RunMetadata()
with tf.Session(graph=tf.Graph()) as sess:
    K.set_session(sess)
    net = MobileNet(alpha=.75, input_tensor=tf.placeholder('float32', shape=(1,32,32,3)))

    opts = tf.profiler.ProfileOptionBuilder.float_operation()    
    flops = tf.profiler.profile(sess.graph, run_meta=run_meta, cmd='op', options=opts)

    opts = tf.profiler.ProfileOptionBuilder.trainable_variables_parameter()    
    params = tf.profiler.profile(sess.graph, run_meta=run_meta, cmd='op', options=opts)

    print("{:,} --- {:,}".format(flops.total_float_ops, params.total_parameters))